I actually got my swap discs from a place called techloops. Came fast and worked well. Back then I paid about $30 for my set but the price seems to have gone down to around $21.
DO NOTE that there is two major types of swap magic: 3.6 and 3.8. 3.6 can't load dual-layer games and may be troublesome with non-dummy-trimmed single-layer games but ultimately will work with any and every single layer game on the PS2. 3.8 works with dual layer and adds cheats but lost some of the single layer compatibility. Since I already own the god of wars I didn't feel any need for dual layer and went with 3.6 myself. The two types usually cost the same because they're just as easy to make; the choice is ultimately up to you.