I would suggest measuring the diameter of the prop and then getting an equivalent diameter prop from another manufacturer.

Often GWS props can be substituted on the smaller foamies Nitroplanes sells, and APC "E" series or "slow flyer" props for the larger foamies.

If the included APC prop adapters don't fit the prop shaft, you may have to drill or ream out the prop with a prop reamer, but this is pretty standard RC fare.

If in doubt use a 6 pitch prop of similiar diameter.

If you know the motor's KV rating, post it as from that ( and the battery voltage ) I can help you figure out a good pitch number.