Rubber seals for doors and windows seem to be a constant pain, and if they are available they tend to be really expensive. Regardless of make and brand.
Old Land Rovers, late 70s onwards, are a dream to get parts for. At least from the UK, and given you can live with aftermarket parts.
Yeah weatherstripping is horrible, but there is an aftermarket for that, too.
Dorman (USA) makes a lot of these kits, you can often get all sorts of surprising parts from them, but they sell out their runs, too.
Thing about that kind of part is there aren't a lot of sales. There usually isn't millions of people out there ready to buy the weatherstripping, so the aftermarket often won't do a production run for it as it can be rather unprofitable.
Now and then a supplier will start making something crazy though, a few years ago I was looking for body sheet metal repair parts for a w123 300d--and found them! Uro production body sheet metal was available domestically for this 35year old jalopy.
Can't find the parts anymore I believe (I haven't tried) but there were so many of those w123 and w124 globally that people wanted to glue em' back together. Basically 100% of them have serious rust problems, seems to be mainly because of the way the sunroof is designed. I digress.
Same for body panels for old Land Rovers and Range Rovers. The portions that tend to rot are readily available. I bolted in two new front inner wings in mine.
Heck, even weatherstripping and carpets are getting available again! Quite expensive so. Still cheaper in some cases then simple repairs in more modern cars. If you can do the work yourself that is.
Old Land Rovers, late 70s onwards, are a dream to get parts for. At least from the UK, and given you can live with aftermarket parts.