Tips for doing this/gaining/developing these skills:
It's not usually a one-and-done thing. You need to train yourself to be an opportunist and think outside of the retail boxes
Make a list of every single thing you are looking for, whether it's an arm chair or "something for the wall above my bed," and then keep a look out for any opportunity you might have to Get the Thing:
Make some kind of blog or pinterest or vision board for the kinds of things you're interested in, so you can start figuring out what you like and really developing and creating a personal sense of taste
Make two lists, saved somewhere you can easily add to them: (1): Things you need (e.g., a new chair, a new outdoor table, a bunch of wall art) and (2): Things that would be nice to have (e.g., cool shit, also a bunch of wall art)
If you live somewhere in a city where people put old/free stuff out on the curb (ime this is most places since COVID), keep an eye on the curbs and corners whenever you're driving somewhere or going for a walk
Make a habit of visiting thift stores every so often (esp with friends), even just a couple times a year. You have to make this a habit because it's not like a regular store where you can basically Guarantee whether they will have what you need/are looking for a lot of the time - so keep in mind all the areas in which you might want to Get Something, and keep checking back regularly (that sounds so tedious if you do this right it's actually really fun
Hole-in-the-wall vintage and antique shops will often have some really good pieces for surprisingly cheap
Literally look up all the stores in your area using the keywords: thrift, vintage, reuse, consignment. Keep a list of them saved somewhere so that when you're in the mood to hit something up, you have easy options ready at hand
Even better, if you can - look up stores using those keywords for the nearest wealthy area(s). "Consignment" and vintage stores in those areas will often still be way fancy and pricey - but VERY few things work better than hitting up the three or four nearest Goodwills and Out of the Closests in a rich neighborhood. That shit is so much nicer and more plentiful a lot of the time
Generally, the less something looks like an Apple store, the more successful you'll be
Outlets are also cool. Little known fun fact: Nordstrom has a furniture outlet called GDF Studios. So shit will sometimes be like: at Nordstrom: $700, at GDF Studios: $150, at GDF sale section: $85. If you look in the right places you can find arrangements for a lot of these things
Similarly places like Ross and TJ Maxx that sell leftover stock from other brands for a discount are GREAT and absolutely worth hitting up regularly
Pick a weekend and hit up some garage/estate sales. People will post ads for these on Craigslist and street corners. I do not do this because I don't do mornings, but my aunt who's an artist will sit down with the paper/Craigslist/her facebook groups, circle four to six garage sales she wants to check out on a particular Saturday or Sunday morning, save the details, and then spend the whole morning going to them one after another, bam bam bam, with a list of exactly what she's looking for
Learn how to do some light DIY and redecoration/repair shit yourself (note: I am NOT talking about "make your own cabinets" I'm talking like. "You can fix the scratch marks on your black dresser with a sharpie" and painting your own room or installing your own curtain rods or something
Bribe your friends into helping you paint rooms/haul furniture/etc. with fun times and/or pizza, as is tradition for moving in general
Keep an eye out for creative uses for things. A lot of shit is priced by what it's for and who the target audience is. So a 7ft x 7ft tapestry or wall art piece will probably be expensive (at least if you don't want it shitty and/or dropshipped). A 7ft x 7ft cloth shower curtain with a cute pattern, on the other hand? $20 at somewhere like Target, $10-12 at Ross or something. (This is how I got the giant plant art tapestry above my bed that I get many compliments on)
Keep an eye on your local "free" and "furniture" and "art" sections on Craigslist (be on the lookout for scams tho)
Join the Facebook group for your local branch of Buy Nothing or Free & For Sale (note: things on this often go very fast)
Depending on where you are, your local library or another local organization (e.g. a Grange, 4H, or rotary club, if you're somewhere moral rural)
Church thrift event days are great actually, because it's concentrating stuff from so many people
Get involved in local fibercraft or traditional craft groups, esp sewing and/or woodworking. This will connect you with contacts, resources, ideas, possibilities, and - if you get regularly involved enough, esp in a group that's mostly older (which many of these are) - access to stuff people are clearing out of their own stashes
And finally, the biggest, best, most valuable and effective tip, especially for bigger overhauls and straight-up furniture:
Pick the nearest university or college town, especially one where a lot of students live on campus. Look up what date their finals week is. Drive around student-heavy neighborhoods during the week of and after finals. You WILL find some really nice fucking furniture on the curb, I promise
The end of the college/school year is also the best time to check sites like Buy Nothing and Free & For Sale, because people are getting rid of things while they move, and in some cases - especially near nice universities - the things they put on the curb are very, very nice indeed