A DIY cube differs from a "regular" cube in that you put it together yourself, so you can adjust the tension to your own specifications. You can't adjust the tension on store-bought Rubik's cubes.
You can purchase a regular Rubik's cube at any local department store or toy store. As far as I know, the only place to purchase a DIY cube is online.
Both cube4you and PuzzleProz sell the same DIY cubes. Here are links to both websites so you can check them out.
http://stores.ebay.com/PuzzleProz
http://www.cube4you.com/index.html
Andrew is right - Puzzle Proz is dependable, fast, and the shipping charges are reasonable since they ship out of the United States. Cube4you seems to be priced lower than Puzzle Proz, but that's offset by higher shipping charges since the product is coming from China.
Another place you can purchase a DIY cube is from
www.Rubiks.com.
Here's a good idea of what it will cost you from each.
Puzzle Proz:
Cube = $11.99 (Comes with stickers - free lube if you purchase 2 or more)
Shipping = $2.99 (3-5 days)
Total = $14.98
Cube4You
Cube = $7.70 (comes with stickers)
Shipping = $20.40 (5+ days)
Total = $28.10
Rubiks.com
Cube = $10.99 (comes with lube & stickers)
Shipping = $8.99 (5 - 6 business days)
Total = $19.98