What makes a game accessible for you is completely dependant on your own abilities. The following list gives some initial pointers. Until there is a ratings system for accessibility in place, you will likely have to rely upon trial and error to find games to suit.
Please feel free to e-mail for help on anything that is unclear.

This list is not conclusive, but should help you track down a good range of accessible games.

SpecialEffect - GameBase of Accessible Games.
IGDA GASIG - many more links via their Blog too.
Game-Accessibility.com - Articles and more.

Audio games - Games for blind gamers.
AbleGamers - MMORPG adventure games.
Deaf Gamers - Review site for deaf gamers.
Eye Tracker games - Eye Tracker games.
Head Tracker games - Links and help.
One-switch games - Switch gaming.
UA-games - Universally Accessible games.

Using eBay is one of the easiest way to track down older commercial games no longer stocked in high-street shops.

UK charity AbilityNet sell the "AccessFUN" memory stick for around £16.99. It is chock full of accessible games and activities.

Sold at cost price, covering research and development.

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Other Video Gaming Shops

Amazon - A good on-line source of both old and new games.

Play.com - Great prices for new games in the UK.