The only widely available DL +R DVD's that we recommend are Verbatim.
If you want the 8x variety Amazon has them here.
The 2.4x-8x variety in a retail package is slightly less expensive and can be found here. You might wind up with some made in India, which many people say are inferior. The best Verbatim DL media is made in Singapore, and the UAE made ones are also good.
Other sites to look for them include rima.com and supermediastore.com.
The only other type of DL DVD I'd recommend are the Falcon FTI 2P process discs which you can find here. They seem to be slightly more expensive and don't have free shipping.