Some of my friends are
massively into hacky sacking, and one of them got into making his own hacky sacks. He got the patterns from a website... I'll have to ask him for the address, because it was pretty good... but I found a similar page with instructions and a pattern for a 4-panel hacky sack:
http://www.valinet.com/~dbotkin/sew.html
They also briefly mention how to do 12- and 32-panel sacks, which I've heard are better for hacking with (the more panels there are, the rounder the sack tends to be - ie. easier to control).
Two tips - the "weak point" of my friend's hacky sack was the stitching, it came undone after a few weeks! I'm not sure how you'd get that stronger though. You also need to use a good strong fabric/leather, since it's going to have a lot of wear and tear. For that reason I probably wouldn't recommend a balloon - that'd be okay for juggling, but shoes are a lot more solid than hands!
Edit: Meep, sorry! I only just noticed that you said you didn't really want to buy leather
I guess it might be a bit expensive... but you might be able to find a cheap jacket made of strong material (leather or otherwise) at an op shop or something?