It can be pretty easy to find clear quartz at your local rock shop – cheap too.
I often dig through the amethyst bins to find that one cheap stone in there.
Ebay can sometimes have some decent rough but it’s an expensive habit to get into and you’ll soon graduate beyond buying rough unseen. (Don’t buy overseas!)
There’s always glass – I know a few folks who practice on marbles and love it.
That being said, of course you should support your local facet grade rough supplier whenever you can. I just can’t afford to pay top dollar for something that I’ll never sell, ever.