The "Boom Brake" is made by Dutchman and can be found in just about every major boating store for about $300. They are very easy to install.
I agree with Jeff about sailing downwind and while a preventer is the choice of many sailors, I don''t really like them, in some situations. To rig a preventer, you have to go up on deck (sometimes this is not desireable) and, in confused wind situations, they may need a lot of attention. Once, a friend was using one and it whipped back and forth and cracked the glass in one of his ports.
If I were to get something, I think I would try the Boom Brake first.
Hope this helps.