If you enjoy the forum please consider supporting it by signing up for a NES Membership The benefits pay for the membership many times over.
Be sure to enter the NES/MFS July Giveaway ***Smith & Wesson SD9VE***
Since you asked distributor and not retailer...
Lipseys or Sports South. My experience with Sport South is if I order on a Monday i will have it by Thursday or Friday including the form 3 processing time. Lipseys same pricing but can take up to two weeks since they only ship class 3 on Fridays and have one person processing all their ATF paperwork. Lipseys tends to have slightly more inventory.
There's no such thing as cheapest distributor that I've found, they seem to all be pretty close on cans. Check around and just keep an eye open for sales.