There’s a whole host of good, useful stuff out there on the internet for designers and illustrators. Here are seven sites I’ve found pretty useful:
Creative Market : Full of lots of fonts, Photoshop brushes, vector files and mockups
Pixeden: Lots of mockups, textures, backgrounds, icons etc.
Graphicburger Mockups, backgrounds, text effects, textures and some UI stuff.
Envato: Loads of stuff, Mockups, templates, audio files, code, scripts etc.
FontSquirrel: Free fonts!
Dafont: Lots of fonts
MyFonts: More fonts