For materials, it depends where you live. If in the US there are a few companies like Seattle Fabrics. I'm in Australia and I use
https://www.extremtextil.de/en/, shipping is very expensive so buy a decent quantity. For a more budget option, you can go dumpster diving at any large outdoors store, they're always throwing out returned or damaged goods, and you can get endless material supplies that way (plus perfectly good outdoors equipment). That's been a goldmine for me.
For materials and ideas, there's loads of info on these forums, scan through an all your questions will be answered:)
This is an easy place to start with bag making, never used it myself but lots of good resources here:
https://www.learnmyog.com/index.htmlhappy sewing, it's amazing and very addictive!
