Is there a mini itx case that I can get that will fit a 400-500w power supply and a gtx 750 that I would be able to fit in my backpack?
The Lian Li pc tu100 comes to mind, though you'll have to use a SFX powersupply and a slim cpu cooler due to how small it is
depends on the size of your backpack. id check out the hadron air, the node304 and the ASRock M8 (barebones)
Silverstone RVZ01 looks good. You'd have to go with a SFX powersupply and slim cpu cooler, but you could fit a full-size 2-slot graphics card in there (780ti if you wanted). Here's a reddit build complete that I found.
Galaxy sells a low profile version of the GTX 750 that you may want to look into. It would enable you to use an even smaller low profile mini-ITX case.