All use seatposts in the early to mid nineties came in one standard size which was 25mm.
USE made the thermoplastic shims of all diameters so you could fit it to your frame, i have a couple of spare shims lurking around, whats your internal seatpost diameter?
edit: just looked on their website, they are £6.99
Below is a better explanation of how they work (taken off the website)
"The USE shim system allows you to fit a post into virtually any frame size. USE shims are made to 100mm length and are longer than other shims on the market. This supports the seatpost inside the frame. All shims are designed with a "top lip" to stop shim sliding into the frame. Depending on the thickness of the shim, it is made from either Rolled Aluminium, Machined Aluminium or Thermoplastic".