Yes; most small parts on GA planes are off the shelf items. You are not going to spend £100k on a diecasting tool for some silly little door handle.
The seats in my TB20 are ex Renault, albeit with custom upholstery.
If the door seal is of a constant cross-section all the way along, it is likely to be a standard automotive neoprene rubber extrusion - especially on a Piper which is a pretty agricultural company. However it might be next to impossible to find the actual part. I have tried this sort of thing (in a different context) and even though I had the actual mfg part # (it is stamped on the part) and a photo, the manufacturer (a huge car parts maker) totally refused to communicate, and many many faxes sent to their various dealers also drew a blank, until eventually I found out that I could order it easily if I had the make and year of the
vehicle it was used in
A dare say a lot of digging around the motor trade would do it but how much time do you have?
An added dimension is political: a lot of these items are unsuprisingly no longer made for the original car application, so the mfg knows that any enquiries they get will be from pilots (or similar) trying to bypass their cushy aviation-paperwork cash cow. There are so many people trying to bypass this system (for obvious reasons) that the whole business is very wise to this kind of stuff and they close ranks faster than the Met
The irony of it is that the mfg rarely produces the aviation paperwork; they ship batches of the item, with a simple Cert of Conformity, to an FAA Repair Station somewhere, which generates the 8103-3 forms using some extremely expensive laser printer toner
Then, an EASA 145 company buys these and generates the EASA-1 forms using an even more expensive laser printer toner
I had some fun trying to bypass Socata on the ISO-thread hydraulic hoses. A £600 Socata hose can be bought from Saywells in Worthing for about £200 (the US-thread ones are about £60). But the mfg of the end fittings (Eaton) sells them only to Socata (and probably Airbus) and sees this from a mile away, and quotes a crazy lead time. You can still get them... with an EASA-1 form. Just have to plan ahead.
It can be fun sometimes, however
Door seals are likely to cost a few hundred quid.