as far as i know, FBFs are an oddity. they aren't used anywhere near as often as RBFs. in fact, one glassblower called them a specialty item.
so, owing to their rare nature, you will be less likely to find them at auctions. also, you are less likely to find them in glassware catalogs (which is where you are most likely looking for them). in fact, a HUGE glassware catalog has 5 FBFs, all 1 neck boiling type (only 1 had ST GG joints). of the 8 multiple neck type, 0 were FBFs. this is due to the fact they are classified as distillation flasks, which should NEVER be flat bottomed.
this leaves 2 options. get a custom flask made, or get an existing RBF converted to a FBF (i believe baalchemist has some experience with this). or stick to RBFs.
the major disadvantage to RBFs is the increased problems with stirring. a FBF distributes the weight of the Al foil over the flat bottom. an RBF has more weight focused on the bottom and therefore the stir bar. also, it is harder to get a larger egg shaped stir bar into a ST 24/40 joint.
the obvious advantage to a RBF is that it can double as a distillation flask, thereby eliminating the need for a 2L FBF and a 2L RBF.
if you have a 2L RBF, try stirring the 27.5g Al foil in methanol WITHOUT Hg salt, ketone, or nitro. if you can get it to stir, and can keep it stirring (very important, try keeping it stirring for 3h at least) then you have what it takes to do a AlHg Nitro.
i learned a thing or two from charlie dontcha know.