You could easily Make one out of 1 1/2 inch steel tubing, or Steel conduit. Probably be cheaper than something made specifically for pull ups, especially if you have an achetectual salvage store or co. in the area.
Then Its just a matter of securing it to hold the weight which could be a number aof ways, something as simple as chunks of 2x4 drilled to fit the conduit attatched securly to open rafters, etc. and a cap on the conduit to keep it from sliding out of the 2x4's.
Just an idea for a DIY project. IF you dont have an area with open rafters or floor joyce though it may be more difficult to make sure you have something to attatch it too securely.