First, go to
Hobie Class Assn. Website and get the line lengths and diameters. (Resources > Technical Information)
You can go one size smaller on the mainsheet (to 5/16") and on the jibsheet (to 1/4") - that's what comes on the new boats and they run through the blocks easier.
Then download the Hobie HOTLINE from Mar/Apr 07
here. The "What's My Line?" article on page 12 explains all the differences in line choices.
You can buy line from Rick's on-line store (this site) or
Annapolis Performance Sailing or
Layline.
I like the bargain bin at APS - I've gotten a couple of mainsheets from that for about half price.
You can spend less than a $100 or several hundred dollars replacing your lines. The most expensive ones will be the mainsheet and the jib halyard on an Aussie system.