@Adrian bolton You may be able to buy a standard base board in Marks Models if they still have them in stock, if you are going for non standard to fit into the attic you would need to know your track plan and build the boards to suit.
Marks Models used to stock these helpful booklets, which could give you an idea on how to proceed;-
Give them a call to see if they are in stock?
There is also a wide range of books online which are very helpful, some helpful members on here should have more recommendations.
Eoin