Unfortunately, you'll need to spend some money to get stock. A good place to buy solid grosgrain and some polka dot grosgrain is Gifts International. Prices are very reasonable and so is shipping if you live in the States (I live in Canada).
Also, to get your name out there search for craft shows or Mom to Mom sales. The first mom to mom sale that I did I made way more than I expected.
Really it's your judgment on what you need but I suggest starting with Gifts International for ribbon
I hope this helps and good luck!