You can use any brand for F18 - Just spent the winter going through the sail purchase process.

Here are the lofts I would look at first based on my research:

Glaser - USA CA, having designed/made sails for 14 years including my own torando sails - I am very happy with what we just took delivery of. Very responsive to our questions and input.

Other options (and have history with Tornado/F18 Development):
Tony Tio - Quantum Spain (trying to get a kite and jib to test).
Zuccoli - Ullman Italy (build many of the European sails)
North UK - I think they are now doing all the Shockwave sails and made sails for one of the top UK Infusion teams.
Performance NED

I did not look at any of the AUS sail makers (Goodall?) but they may be a good option based on the exchange rate.

Do your research, ask lots of questions, watch the euro if purchasing from overseas....