Finding a used NACRA 18sq sail is going to be difficult. There were delamination problems with the factory sails. That means there are not many left.

My first option would be to by a new sail. If you expect to keep the boat for 2 or more years it is probably worth it. You will be amazed at the difference a new sail with make. I know Ullman in the USA and Europe has patterns and experience.

If you can't or do not want to spend the money there are a couple of options

A 5.5 uni sail will fit. It will be smaller, 16 sq m instead of 18.

You can also have a sailmaker recut a sail. Woven sails can be recut. Laminated sails in general cannot be recut.

A section will be probably have to be added to the bottom of the sail to support the trac. The leach will have to be reshaped. The sail probably be too full for your boat if it was intended for use with a boom.

It will cost $200-250 if you take it to a sailmaker offically. A person working in the shop, doing work at night, might charge $75-100. Ask around to see if anyone is doing this type of work in your area.

A good source of sails for recut are used Tornado sails. In general raced out Tornado sails are still in very good shape. You will want a dacron (woven not laminated) sail.

I have a rule about catamarans. They always cost at least $3000. You can spend $3000 and have a good boat or you can buy a boat for less and spend $3000-4000 total before it is in good shape. For example a $600 boat needs new rigging, new sails, trailer repairs, etc

Carl