Dollar tree surprisingly has SO MANY witchy supplies you can buy for just a dollar each. I have the privilege of having a local dollar tree within walking distance of my house that I go to often for supplies. Here's a list of things I tend to buy.
1. Candles galore! Omg I have so many candles from dollar tree. They are quite small and would probably burn down in a few hours, so if you're the type of person who lets a candle burn all the way, it won't last long. I tend to get the packs of 20 tealight candles (which is a steal because it's literally $1 for TWENTY), and the candles in jars. There are many other options though such as pillar candles, taper candles, birthday candles (great for a quick candle spell), prayer candles (those things are big), etc. These are great because there is a large variety of colors and scents that you can use to match your intention. Idk if the chemicals used to make them are good but I don't really care. There are also other supplies for candles such as plates to hold them, candle snuffers, and lighters (which you may not be able to buy if under 18...).
2. JARS OML! They have so many packages of mini jars!! They can come in packs of 6-8 I think depending on which kind you buy. There are 3 varieties of mini jar sizes at my dollar tree at least. There are some with corks and others have metal lids that you screw on. Great containers for herbs and spices as well as spell jars. There are also larger jars, but I would not recommend putting water in the ones with metal lids cuz when I did that, it rusted shut and I couldn't open my moon water :'(. There are large jars with cork or plastic stoppers that I have found, but I have heard that they can contain lead, so please be careful and it's probably best to not drink out of them.
3. Vases and bowls. Vases are obviously good for holding flowers. Bowls have so many uses. You could put (moon) water in it and place it on your altar to represent the element of water, you could grind up herbs or other spell ingredients in it, you could use it as a cauldron and so much more.
4. Incense. Incense is amazing (if you don't mind smelly smoke). It can represent the element of air on an altar, as well as cleansing and many other things. There are also things that could be used as incense holders such as a feather shaped plate I found in the incense section. I've also seen people buy dollar tree incense that came with a holder so keep an eye out for those kinds.
5. The spice section. The spice section has salt (which you will definitely want), basil, thyme, pepper, and other herbs/spices. I've also found pink himalayan sea salt near the bath section.
6. Containersss! There are small and large plastic storage containers (with lids) as well as wooden and cardboard boxes (that you can decorate) in the craft section. There are so many containers that you can find that work well for storing (or hiding) your witchcraft supplies as well as mini travel or hideable altars. You can never go wrong with containers.
7. Notebooks. There are a variety of notebooks that you can buy. They work well for note taking, a Book of Shadows, or a Grimoire. You don't need anything fancy, and a regular looking notebook would be the best option if you are in the broom closet. Be careful that you buy a sturdy one though. If they are held together with glue, they're probably not going to last long before falling apart. Sewed or spiral notebooks are probably the best ones. I bought a pack of three mini (sewed) composition notebooks which I use as manifestation and sigil journals. (Mini is better if you plan to hide it).
8. Bath/Beauty section. Self care is important, and there are many self care and beauty products. I haven't really tried any of them though lol. Idk if they're the safest since they're so cheap so be careful. But the bath salts are great. There's a lot of great products to use for bath rituals and such. You could also buy makeup or skin care products and enchant them by anointing them with moon water. Take care of yourself!
9. Seasonal stuff. There's usually stuff for whatever upcoming holiday there is. For example, right now it's nearing Christmas (Yule) so there are many great decorations you could buy. You could buy stuff and use them for offerings to deities, buy stuff to use in rituals on the sabbats, etc. It all depends on which season/holiday it is. But you can most definitely find something useful for your craft.
10. THE CRAFT SECTION is awesome! So many useful things there. Glitter (which can be useful for color correspondences), paints, brushes and canvases if you enjoy incorporating magick into art, the wooden/cardboard boxes mentioned earlier that you can decorate, felt sheets and model clay which can be used to make poppets and spell dolls among other things, string for knot magick, etc. You get the idea. Witchcraft has the word "craft" in it for a reason. Be a creative witch.
Disclaimer: not every dollar tree has all of these products that I mentioned. It all depends on what gets shipped to them and when. If your dollar tree doesn't have a product that you wanted, don't be discouraged. There are other ways to buy it or even make it. Remember that the most important thing in your craft is yourself, everything else are just tools to aid you. Also, the products may not always be the greatest quality. Buying things cheap is great, but in the end it's the quality that matters.
Hope this was helpful. Don't forget to comment product or other store recommendations!