If it's for RS, nothing can be guaranteed about long-run durability, especially holding zero. For airsoft there's quite a lot of possibilities. Have a look at DragonRed Airsoft (
http://ww.dragonredairsoft.com/), they have a fairly large selection. EbayBanned also has a bunch, and shipping is included with the price. If you're around Toronto (BTW, it would be helpful if you could fill in your profile), Huang should be able to help you out.