Did the Mac that you bought not come with an installation disc? If that is the case, you can probably buy one of the earlier releases (OS X 10.3 Panther, for example) of the Mac OS. Check out eBay. You are looking for a retail copy of the operating system, not a copy that came packaged with a Mac, if possible, since the discs that come with the machines are tied to the models with which they come. Now, if someone has your model, and will sell you the disc, that should be fine, but getting a retail copy removes any risk.
I suggested an earlier version, because I assume that Panther will run a little faster on his model, but someone else, with more experience with G3s, can suggest buying OS X 10.4 Tiger, if they feel it will run just as well as, or faster than, Panther.