There are many threads on this subject for you to search for but one of the best is to get a hollow core door (good size and flat) and set it up on legs. Then use a piece of ceiling tile for the actual (pinnable) building surface. Pin into the back side of the board (no holes).

Very important to have a flat surface big enough for your project. A piece of Melanine wood can also be used but may be hard to find one big enough. You may be able to find a damaged door (one side) at a reduced price. Never hurts to ask.

If you know you are going to be continuing to build, there is the magnetic board system.
http://www.airfieldmodels.com/inform...oard/index.htm
Very nice but not cheap initially.