bawbag,
Where are you located? Most marine supply stores will have these in stock or be able to get them for you.
I purchased mine a few years back from Port Whitby Marine Supply. http://www.weblocal.ca/port-whitby-marine-supplies-whitby-on.html
For 238 extension post reference, http://www.attwoodmarine.com/store/product/Bell-Pedestals-Fixed-Height
For 238 series seat mount reference, http://www.attwoodmarine.com/store/product/238-seat-mounts
When choosing a length for a fixed post, make sure you take into consideration the height of the seat base/mounts and the height (thickness of cushion) of the seat you choose to make sure the total height isn't too much.
Not sure of current pricing. I remember paying around $100 for the fixed post and base combo and almost double for the adjustable.