Several on the MOC seem to have an affinity for Trekwood for the go to business for RV parts. I know they have a good inventory of both common and off the beaten path stuff, but there are other companies worth a look see. Steele Rubber products and RecPro may have the seals in question here as well. Most flapper seals are either 2 7/8" or 3 3/4" with the latter used mostly on top of slides and the shorter used on the sides of the slides. The bulb seals vary as well from 7/8" on up a bit. Best thing to do is measure what you have and order accordingly.
One personal experience about ordering on EBay or Amazon ... you get what you pay for. The seals may seem like in kind to yours, but they are not made from very good material and don't last like you'd expect. Been there, done that, have the T shirt.