The front seats in my van are feeling a little elderly, and trips from London to Devon require a butt-massage on arrival

Does anyone know a good firm to get complete seats from? JK doesnt seem to do seats.
And any tips? Do I replace the seats including runners? And is the base a standard size, or not?
And how easy/expensive is it to get and fit swivelable seats? is it a DIY job?
Any help will be much appreciated by me and my equally sore passenger!
cheers,
matt