Metalsupermarket is a very convenient / easy to use (though somewhat expensive) supplier. The one near me sell "drops" (the off cuts) for reasonably cheap.
It is sometimes worth it to learn on paid materials rather than trying to figure out what the hell is wrong with your welds because you don't know what the material is, or it's heavily contaminated and inadequately cleaned, etc. If you're struggling mightily with a certain junk material, I'd consider jumping to a different material after standard troubleshooting, but before wasting tons of time trying to wrack your brain to figure it out.