I buy things from all over. I’ve bought bulk pieces from the Game Crafter at conventions, large packs of cubes, bags of random meeples from MeepleSource, etc. For the past few weeks, I’ve been trying to collect them all in one place (a couple of stackable bins) so I have them available for my next prototype.
I use a lot of different things. I started my career in design so I’m really visual. A lot of time I will spend an hour and design some tokens or player pieces that fit the theme I am going for with the game. Probably overkill but it works for me. I will also pull pieces from other games I have in my collection. Finally, I keep a box full of random tokens that I 3D printed or found on Etsy or EBay.