Buying New Line (rope)The references for buying new line are somewhat confusing, some references are listed in millimeters and some are contradictory for what size for which line. So I'm going to simplify that problem here. (also see the conversion chart below.) . But remember, if you're racing you must follow the official specification for all line, sails, trim, boom, mast, etc.
The 25' halyard haul line (hoists the sails) should be 3/16 (US available at West Marine) which is approximately 5 mm but you could get by in a pinch with a nice 1/4" line if you wish, as long as it is strong. In this case, the smaller line diameter works better since we're threading through a small pulley and tying off to a small cleat. See http://www.westcoastsailing.net/default/boat-parts/sunfish/lines/halyard.html
The mainsheet can definitely be 1/4 instead of the 7 mm to 8 mm specified in some of the catalog sites. Diamond braid from Gander Mountain or other sites worked fine for us this weekend.
I think it is a good idea to use different colors for the different lines. This will avoid confusion. Also, I'd avoid nylon braided rope for either of these two, preferring instead a nice climbing type line/rope that has an outer sleeve.
We had trouble getting a new sail on-time because the "All-American" we ordered is apparently very popular and ended up on back-order. We were lucky enough to have the seller, the Sail Store, offer us the temporary use of a plain white sail (with window) (The FX sails are really a nice quality!). We took them up on the plain white loaner and it worked fine. Since it was temporary I avoided using the shower curtain hoop-type attachments. Instead I rigged the sail up with one very long strand of parachute cord for each boom, tying special hoop-like structures as I went along the length of the booms. This worked quite well. I'd guess a good rope would hold up pretty well if you're in a pinch, but may not be the best permanent solution. I think it looks better and will hold up enough for our use. It'll also be easier to de-rig when the All-American sail comes in.