Not sure what the carb heat seal ass is, but if its the little valve that diverts the hot air off of the muffler, it's a weirdo part unlikely to be at Spruce or McFarlane.
If it's a part not subject to wear, you could hit up any number of aircraft salvage yards -- we use Kenny Faeth because we're on the West Coast, but Cessna Pilots Association has a nice list of them nationwide.
If its the butterfly valve assembly, I recall Wag Aero or similar shop sold a rebuild kit to replace the shaft and the bushings only, which effected the repair, and it was inexpensive. This was 5yrs ago, not sure if this is still available.
Your mechanic gets a 20% discount through most Cessna houses. If you're doing this "as preventative maintenance", then snuggle up to your local FBO and have them order the part for ya.
$0.02
- Mike