For upgrading seats the following options are generally available (The first 5 pictures taken from the online MGOC Accessories guide):
Any number of companies seem to offer covers in a large choice of vinyl, vinyl with cloth/nylon facing, or vinyl with leather facings. Colour options, including contrasting piping is numerous and can be offered with matching trim panels and centre console box lid.
Fitting new covers can be done with the seats in position, but it is probably a lot easier done out of the car giving you a chance to repaint the frames. A little trick is to fit plastic over the seat back to make fitting the backrest cover easier. It is also advisable to fit new foams and rubber panel to firm up the seats, but this will raise the seating position for a while until it settles a bit.
Note that the seats changed in 1970 to incorporate a recliner lever rather than the adjustable bolt method of earlier cars. Seat base foams also varied in design between c/b and r/b cars. head rests also changed to a flatter shape with the r/b cars.
A number of suppliers also offer luxury specification seats with a more modern recliner wheel and twin post head rest. These are more supportive with more pronounced bolsters and come in a variety of colours and facings. The seats are not cheap either but are a good upgrade for a no-expense spared rebuild.
For the really supportive seat in motorsport events then the bucket seat is the one to go for, whether a classic looking bucket or a more modern unit with holes for the 4-point harnesses, or even with wings to support the helmet. Of course, these work well with a Roadster, but if fitted in a BGT, they generally do not have a forward tipping option so access to the battery will have to be via the boot.
These usually require a separate sub-frame to be purchased, and also some may be too wide to fit the very narrow floor pan of the B - the maximum width you should aim for is 460mm width of seat base.
The Rear Bench
One problem with the rear bench is the fact the battery bins are located below, thus raising the actual bench height. This is an interesting modification with the battery moved to the engine bay, and the seats lowered, allowing taller children or small adults to squeeze in.
This is another option if you have the skills, such as the one detailed here using the existing frames or these that I saw on an Australian car belonging to Hans Pederson in Melbourne.
This one below is my car with MG ZR seat covers and foams fitted to the existing frames with modifications for the head-rests.
This one below is a car belonging to MGOC member Paul Johnson showing his ingenuity in fitting the MGF seat backs to the B seat bases. Paul has mentioned repeating the exercise in the future and photographing the process for me so check back for an update.
Seats from Other Cars
From the various bulletin boards, it would appear that there are two main options. the reason for such a limited choice is the restrictive width of the B floor pan.
Fiero is the most common in the US, but also available in the UK, and covers to refinish them can be purchased to suit any choice.
Mazda MX5 (Miata) are also a suggested fit.
In both these cases, apparently headrest speakers can be fitted, and modifications are necessary to the seat bases.
This is a set of Renault 25 seats in an MGB. These look particularly comfortable but are possibly a little disproportionate to the B's cockpit in size.
And finally, one option that we have yet to prove, but that is looking a very strong possibility are BMW MINI seats!
It probably means removing some of the plastic surround from the seat base to fit it in. Anybody tried this and willing to share some photos?
This page is a summary of all the seat types available for the MGB plus a few unusual considerations.