Blackburn Pumps...
 

So, I've had a Blackburn Airtower 4 pump for years. Solid, extremely well built floor pump. Worked great until about two weeks ago. Took it apart, and it turns out that the rubber gasket/piston on the end of the plunger was shot. As Blackburn pumps have a lifetime warranty, they replaced the entire pump for want of a very inexpensive rubber part that they no longer make available.(why I don't know...)

I just received this new $100 Blackburn Core Pro pump by Fedex this morning. Thank you Blackburn.

BUT I still have the AirTower 4 pump that is perfect save for a stupid little rubber part! Anyone have any ideas? I could sure use a spare pump...
https://cimg4.ibsrv.net/gimg/bikefor...6c07ca21e.jpeg

I'd try to find a leather pump gasket with similar dimensions. Measure the pump ID and plug the number and "leather pump gasket/plunger" into Google. Here's what they look like, and other terms you can search under:

Silca 73.1 Leather Washer Plunger Seal for PISTA STEEL Floor Pump Made in Italy 8032532410158 | eBay

Anyway, this is why I got a Silca...still making parts for pumps 50 or 60 years old...

A few bike specific companies sell replacement plungers. The hard part is finding the dimensions of them but probably something will work. Check websites for spares; Silca, Topeak, Lezyne, SKS, Zefal, and probably a few others. Scroll through this place for some examples. A few listed (and a lot of stuff that doesn't apply). Bicycle Pump Accessories and Small Parts - Modern Bike Also pneumatic or hydraulic pump suppliers probably have something. Hell, my mini grease gun uses the same type of washer, just not sure of the size.

Yeah, something in leather may be a little more forgiving dimension-wise. I would think you'd want something a wee bit tighter rather than looser.