I strongly second the above recommendation to try Calvin at brightflashlights.com. He's honest... and very knowledgeable.
I've purchased several Surefires and misc. accessories from him. He has about the lowest prices on the internet and ships fast.
Keep in mind that any brick-and-mortar store/gunshop that sells Surefires is "handcuffed" by their new, rigid (no discount) pricing policy. Brightflashlights, in my experience, usually sells Surefires for about 25% below retail. You might occasionally find a slightly better deal on e-bay, but that can also be iffy.
I urge you to do your homework before you e-mail or call Calvin/Brightflashlights (his number is on his special advertising section on the AR-15.com site) to first determine which Surefire model, parts, etc. you want/need. He's usually pretty busy -- his fame is spreading -- and I don't think he always has a lot of time to answer questions from new-to-real-flashlights tire-kickers.
There are a lot of decent flashlight manufacturers out there (Streamlight, Inova, etc.) but Surefire makes the very best. There's a reason while they're way backed up with U.S. military orders right now: their quality, design/features, durability, and no-questions-asked warrantee service.
Buy your CR123 lithium batteries in-bulk direct from Surefire (they ship slow), or from batterystation.com (the cheapest), or from BrightGuy.com.
Hope this helps.