The Monogram B-17 and B-24 are like the perrenial corner of the garden, they keep coming back every year!
Seriously, though, these kits come and go with some regularity and are always being re-released.
If you can't find one at your local shop, try a mail-order house. As previously mentioned, evilBay is a good place to find kits as well, but sometimes the bidding can get out of hand. Right now things seem to be fairly calm so you could possibly get whatever you may be looking for at a decent price. (Even with the inflated shipping some folks charge!)
Another online alternative would be the various modeling forums. FSM has a separate classified section on this site, so they don't have a "Swap Shop" kind of forum here. But other forums have no such rules and indeed some have specific forums just for selling. There have been three forums that I've used to sell my wares, but at the moment only one is still in operation, the 'Plane Trading' forum on Hyperscale.
If you're hard up, make a post there asking if anyone has the kit that you're looking for.
The last alternative is to find a kit at a model show. I'm fortunate in that every year there are a dozen or so shows that are either really close or within a reasonable driving distance. You may not be so fortunate, but try and find out if there are any shows near you anytime soon.
Fade to Black...