If you're going to make your own sails, the first thing you'll need is a good sewing machine. The type of material you'll be working with and its use determines what type of sewing machine you'll need. Get yourself a quality sewing machine that does a good, zigzag triple-stitch in multilayered polyester and rest assured that your sails will hold against the elements.
Everyone is familiar with the Singer brand name; it's virtually synonymous with sewing machines. In fact, this is one of the best reasons to use one. Not only are you assured of industry standard quality, but also should the machine need refurbishing, then acquiring parts or a repair expert won't be a hassle. The machine to look for is a commercial-grade HD-110 Heavy Duty. Obviously, the Singer isn't the only way to go. There are plenty of good, brand name machines out there, such as, Pfaff's 300 Series, Elna's Supermatic and Viking's Model 21.
The biggest reason a regular, domestic sewing machine won't work well is the material with which you're working. Polythene terephthalate sailcloth, commonly known by the brand name Dacron, is light enough that a common machine can stitch through it without slipping, but not when multiple layers are being put through. Generally, sailcloth is made of at least four layers of cloth, sometimes ten. To handle this much material, it's best to use a commercial-grade machine. Another reason to use a commercial-grade machine is the type of stitch you'll be relying on, namely the zigzag triple-stitch. It's not a complex stitch. Yet with the strain the wind applies to the sailcloth, it's best to have the securest stitches possible. Moreover, the commercial-grade machines are of much better quality than the plastic junk now found in the department stores or on eBay.
Where to get one
It's possible to sew sails by hand, obviously. The Phoenicians and Vikings didn't need sewing machines to get themselves to the New World. On the other hand, their sailors, much like today, relied on sailmakers, experts of the craft. Your pursuits are probably a bit more humble. Get a machine and get the right one for the job. Do some poking around. Investigate. The best thing to do, really, is to find a local shop that does a lot of trade in sewing machines. Chances are that their staff will be knowledgeable and that they will have a wide variety of used machines to choose from within a manageable price range.