www.eurasia.nu sell modchips for $4 (£2) although shipping will probably be $7 or so. They take paypal as well which is handy.
If you aren't upto installing a modchip, the swap trick works a charm. There are several ways to do it. The cheapest way is to bluetack down the lid and swap out an original game for a backup whilst it boots up past the Sony logo but it takes practise to do and if you're like me you'll scratch up your original disc over time.
Next cheapest way is to get a pressed cheat CD like an explorer/action replay disc (I had a final fantasy 9 cheat disc I got free in a magazine for example), blue tack the sensor down, boot up with the cheat cd, cd will stop spinning once the cheat menu comes up. Swap for the backup and exit the cheat menu, the game will boot!
Similary if you have an older model playstation with the data/com port thing on the back you could get one of
these, which work in the same way... just bluetack the sensor down (or use a spring to keep it down), boot with an original game into the cheat cart and swap out for a backup.
A few games such as Gran Turismo 2 won't work with the swap trick because the game will recheck the TOC after the disc has booted, but very few games ever bothered doing this, so on the whole compatibility is very very good.
