When I took out the rear axle/hub one of the rear axle o-ring seals broke. I was going to order from autoricambi but hate to wait for it to arrive and would like to get the rear brakes back together.

Here's the part I'm talking about.
https://www.autoricambi.us/cgi/commerce ... ey=SE6-411

Is an o-ring this size something I get can locally at an auto parts store? Or is this size specific to fiat spider's?

I've had the best luck finding a large o-ring supply at a hydraulic supply house. You will need to take the old one with you for diameter and cross-section. I would be very surprised if you could find one.

This is the best site / description I could find for this part with cross-referenced numbers.

http://www.autopartoo.com/oem/fiat/4160140.html

Maybe someone will have it based on this info.

I think fiat/4160140 is r/w seal not o ring Inside Diameter: 30 mm,1.181 in

Outside Diameter: 45.24 mm,1.781 in

Width: 8 mm,0.315