I need to replace the rubber that forms the seal between the after end of both door windows and the body of the car. Any ideas where to find the seals?

One source is Pelican Parts www.pelicanparts.com
$49.50 each, spendy little suckers. Not too hard to remove/install. The aluminum strip track they ride in is "pinched " in a few spots. Pry them apart and slide old out. Use some sort of lubricant (armorall works) and slide new in and pinch the trim strip again.

Check the PP site and it should direct you to the one you need. Very helpful.
If it is just the end guides they are $25.00 each.
CCLINSEALAND

Seals are items where saving costs might turn out to be expensive.

Reproduction seals might look decent enough, but often don't fit correctly and are sooner worn out. I and many fellow Dutch 914 enthusiasts share this experience.

In other words, get the original items from a Porsche dealer.

a guy on ebay has em...i bought em...pretty good