Hi, don't know of any vendors selling both 510 parts and L88b parts, but if you're just looking to save on shipping, this is irrelevant-- blu doesn't charge for shipping, so anything you buy from them won't up your shipping bill. Buy L88b parts for your blu from blu, buy stuff for your 510 anywhere else. (I like InnoVapor, personally, but you can probably save more money at Rocky Mountain Vapor.)
Incidentally, you can use 306a carts and atomizers on the blu batteries, or even just 306a carts with blu atomizers. I recommend the latter, since I've tried the 306a atomizer and it really doesn't seem to make much difference when used with the same cart and battery. The blu atomizer is actually pretty darn good when used properly and it's the cheapest I've seen, $10 for two shipped free. The 306a carts are the same size mouthpiece as blu's but seem to hold a bit more liquid and seem better-constructed. So far I've only seen them in matte black, however.
Using 306a carts and/or atomizers doesn't affect your ability to use the pack-- everything still fits fine. The only caveat is if you're planning on carrying 306a carts in the pack without atomizers attached, and plugging them with the rubber plugs they come with-- you'll have to shave a mm or two off the top of the plug (easily done with an Exacto knife, I've done it many times) or else the pack's lid won't close. But personally, I find it much more convenient to just cap each cartridge with a blu atomizer. Makes swapping out carts/flavors much easier, too.
