Hi,
I've been looking to do the same thing on my rib - for me it's about making it a bit more family friendly.
There are a few manufacturers but they most bench's seem to be specific to the individual hull, so you might try contacting Humber, another alternative is Leeway, see the link below.
http://www.leewaycomposites.co.uk/
I did find another maker at the Southampton Boat Show - Outhill, but they seemed more expensive
http://www.outhill.com/bench_seating.htm
Good luck and let me know how you get on.
Steve