If your rig is the original on a common production boat, online should be fine. If you have a non-standard/non-original rig, you are really better off going to a nearby loft. Get multiple quotes and see who will beat whose prices or up the quality.
You can often get a better price
locally than online by paying cash -- most lofts(especially the smaller ones) will give you a hefty discount and/or free upgrades for paying cash.
You will also get a better deal in the winter when the lofts have less repair work and need the business. Recutting/refurbishing/restiching could cost you near the price of new; once you buy the used sail and pay to have it worked on, you could end up paying 50-70% of what a brand spanking new would have cost.