eBay. You should get a "retail" copy (which came on three CDs not one DVD), black with a big silver X. Don't get a copy that came with a particular Mac model (grey color), unless it happens to be the disc that came with your particular Mac model.
You may want to consider getting Tiger (10.4) instead, if your Mac supports it. Tiger provides better software compatibility with more recent applications, compared to Panther.
If your Mac is a G4 or G5, 867 Mhz or faster, with at least 512mb of RAM, Leopard (10.5) is also supported.
NOTE: Leopard does not have "Classic" mode for running Mac OS 9 apps. Tiger (and earlier) does.