I tend to use a 22 watt round florescent daylight lamp (about £25 from Maplins when iut's on offer).
I need to replace the lights in my computer room soon anyway, so I'm currently trying to find somewhere locally that sells Osram Lumilux tubes*, as if I can I'm going to put in a pair of 4 foot double tubes (I can buy them online, but last time I ordered 6 foot tubes from Screwfix I ended up with 3 deliveries of about 6 tubes each, of which 3 were usable).
As ghostlypea says wet palettes might work for you, I know my brother likes them but I've never got on with them (I think how useful they are depends on your painting style/habit).
*B&Q used to sell a GE version, but they've stopped doing them (I bought two for the garage and only one store out of my 3 local ones had any, and that was going back 2-3 years).