https://t.me/SaltSoupGarage
This is the telegram channel of Kirlif, who is a very popular modder and active in mobilism. He post his mods in this channel too.
It's very depending on which apk you want. If you only want the apk of one specific open source app, just check on fdroid or on their git (mostly on github). And if you want play store apps without installing Aurora store, just use ApkMirror (their apks are mostly clean, cleaner than Apkpure in any case..).
actually paid $3 for this app like 5 years ago but the fact that it's missing from all the official locations means I'm going to have to pirate it to get my $3 back
If Mobilism doesn't have it, I search Google and try whatever sites come up. If that doesn't find the right thing I just assume it doesn't exist. Depending on what I want, I also try making the apk myself with Lucky Patcher.
Depends, usually searching for the apk with version code (or at best package name) works fine especially when you need specific youtube versions for revanced, or premium unlocked apks.