Omegas who lose their alphas, especially the ones who don't withdraw from the fading bond, can suffer from pheromone withdrawal. This is especially true of omegas before their heats stop. While perfumes lack the actual pheromone component, scent memory can help an omega through the time after losing their mate, and even after that if they so choose.
Naruto Omegas
Haku: He learns how to make his own tinctures and essential oils and invents an unusually accurate combination that triggers profound relaxation in him. Honestly he feels like it's a drug sometimes. It makes his few remaining heats before they stop almost as easy as having his mate around since his body regulates just the way it would based on habit.
In his wandering, he develops a little bit of a reputation for his skill and many omegas will ask him to make a scent for them. He gently refuses until his heats stop, because he's worried about forgetting the exact notes of his alpha's scent. After that, he will occasionally help someone and then it becomes a way for him to live well in every town he stops in.
Kakashi: would be heartbroken if his mate passed away before him, especially if they're both relatively young still. His mask becomes something he uses to keep their scent with him always, dabbing a little perfume on his neck so he still smells freshly scented and a little on the inside of the fabric so he can taste it on the back of his tongue.
Sasuke: keeps a bottle on him always, and often holds the bottle, stroking the sides, touching the cork, pinching the string holding a few charms to the lid, but he will usually stop himself from opening it because it's like being hurt by his mate's loss all over again. He has rough heats after his mate passes away and the imitation scent helps as much as it hurts. As the grief starts to ease, he will sometimes dab a little on his wrists or behind his ears so the scent drifts around him, making him feel like they're always watching over him
JJK Omegas
Satoru: will figure out how to make this himself. It's time he doesn't really have, but it's just not accurate to his alpha's scent otherwise. He tried some expensive bespoke stuff that carried the right notes, but it just wasn't the right way. Once he has it, it stays in his bedroom drawer, and an extra bottle at your old apartment. He's most likely to splash it around there, especially as his alpha's scent starts to fade, to make it feel like someone is still there to make a home for him to come back to.
Nanami: is very polite about wearing his mate's pheromone imitator because he wants to keep all his emotions to himself and also feels it's rude to wear an obvious perfume in public, in case someone is allergic to his alpha. It's okay, this also makes him laugh. He'll take more walks outside so he can wear the perfume at his wrists and collar without feeling impolite. When the remnants of his heats are dying away, he will still spread the scent on his nest bedding, and also on the pillow right by his.
SNK Omegas
Levi: he's sensitive about smells so he almost doesn't get one until Erwin's mate suggests a place that only uses natural ingredients. It isn't the same, but it's enough that when he's alone in his, your, bed, he can set the bottle behind him like a diffusing oil and feel like you're scenting the room for him. It helps him sleep but he often wakes with his heart so heavy he has to force himself out of bed.
Erwin: keeps the bottle on his desk so he can watch the light shine through it. He likes to keep it very light, but all over, as though you may have just walked by, scented the house, run through with your pups when they were young. He uses the scent to trigger happier memories of your time together and then write those memories down so he and his pups can remember his alpha