I've never bought a pre made as it's cheaper and just as effective to make one yourself, not to mention a sense of achievement when it works perfectly. If you're growing 2-4 (like i am), then knock yourself up a 3' x 3' box or something similar (with a height of whatever you want depending on how big you want your plants. (I've used white melamine wood so it's reflective but you can cover surface in Mylar which is about the most reflective material to buy). On the inside of the roof of it I have 2 EZ Rollers (
http://www.basementlighting.com/Merchant2/merchant.mv?page=1/PROD/EZR1). I hang the reflector from this which is easily adjustable so you can adjust as height of plant increases. Lower down on the base of one side, I installed a simple small bathroom extractor fan (but reversed it i.e. flipped it 180° so instead of performing it's job of extracting condensation from a bathroom, it performs a task of bringing fresh air from outside into the grow room). Having constant fresh air, helps the stems grow strong. At the top of the opposite side, I have a vent to allow bad to escape. Inside on one of the walls, hang a thermometer so you can keep an eye on temperatures and get the best for your plants. This is my set up and works perfectly well for my requirements, but everyone is different! I'd say read up, get advice and then find your own path to getting something that suits you - then you'll be perfectly happy with it!