From the photo that appears to be a standard double bulb vertical slide seal. I replaced two of mine. The slide seal is sold by the foot. I measured the length of both seals I needed and then ordered a couple of feet more to be safe. I purchased the new seals from Lichtsinn Motors, a large Winnebago parts dealer. They ship parts all across the USA. Have your Winnebago ID number available and they will confirm the correct seal. I have used them for the last 10-12 years for all Winnebago specific parts.
When replacing the seals, is is easiest if the slide is only opened about half way. Also in your photo you can seal a hole near the end of the seal in the photo, that is from a screw that holds the seal in place at the top. You need to remove that screw from the slot prior to installing the new seal. If you use any caulking to seal the corner seams, remember to leave the slide partly open until the caulking dries.