Drat, mine are 27.2.
You can find oem brand seatposts for cheap on ebay, as sellers dump take-offs or people upgrade their stock seatposts.
Here's a 31.6mm x 325mm cannondale alloy seatpost for $16.25 + ~$9 shipping
I think you should aim to get a road bike for the trainer though, you don't have to pay all that much if you look around. Especially so if you get used.