I’m looking for used seats from a car to fit the front of my series 2a. What is Thebes the replacement?

The easiest option is those from a late model Defender. Ipicked up a pair on a well known auction site quite cheaply. They bolted straight to the existing rails and are quite comfortable enough for me.

Agreed. Defender seats are actually quite comfortable (when they aren't falling apart, that is) are obviously designed to fit in that space, and don't impede access to the underseat boxes. Buy a pair cheap, refurbish them with new foam inserts, plus the covers if they're torn, and put them in. The middle seat may be harder to find, but unless you really need three in front then fit a cubby box instead.