Yep, I'd agree with the Danger Den comments - they're one of the top watercooling suppliers (not that there are many, it's a niche market). I have their original 'maze' block, and I'm using it with the copper cooling cube radiator. There's other stores such as
www.overclock-watercool.com that I'm not familiar with, but also have good reputations. Check out
ProCooling for some great watercooling articles and reviews, and of course there's loads of articles here at oc.com.
Like Shadow says, it's hard to suggest a price - if you want to buy all purpose-built stuff, either individually or as a kit, it'll probably cost around 150 bucks. However, you can save a lot on a radiator by picking up an old car heater core from a scrap yard - again, there's a few articles on using heater cores as radiators here at oc.com. You could, if you have the tools, make your own waterblock, but it probably wouldn't perform as well as the purpose-built offerings from the proffesional vendors.
Also check out
Spode and SurlyJoe's site for some more crazy ideas. My one bit of advice when going watercooling would be to read around a lot, make sure you have a good idea of what's involved - it's not hard to make a good watercooling rig, but it does take some planning and thought. Have fun