ZThere is an amazing thread on this forum to help so definitely check that out.
for in game I feel like there is a “window of opportunity” to get a few. First up, visiting a well stocked party (all hobbies and careers available) when it just opens, if you attend a well stocked party late all you’re seeing are automated sims, the players have likely moved on, however when you attend a fresh party and you see the spammed announcement of ‘X has joined the party’ you know there are players looking at the screen, this is the time to just ask over chat for the sticker you need but make sure to ask what sticker they need, you got to give to receive. Next, at the same party is you want to bring all of your sims, it’s a hassle at times but don’t just bring one otherwise the others never get seen, I found that players give out stickers randomly when they attend a party, if you have populated all your sims in a newly opened party this raises your chances for more random stickers for all of your sims as people are just handing them out like candy. Lastly start filling your sims contacts up, just click on everyone and add the sim to your sims contact list (doesn’t burn any party cake energy) what this does is randomly add your sim to their world so your sim walks past their home during the week or shows up at their work, this also increases the chance of you getting future stickers from them so not only are you getting new players to give you stickers at parties but you might also get stickers from these old contacts... so more stickers! Edit: OMG I need to start reading posts, pretty much what @PlanetAuto has already said 😂
if all else fails many successful sticker collectors get an stunning yet rare outfit from Izzy to really stand out, start investing in Izzy. The rarer the better, I have a in game pal who got the jackpot money fall effect on a basic top, for example, but since no one has seen that effect she is raking in the stickers this week. Start saving those gems.