Unless you've done some fabric work like that before and have all the equipment, I'd recommend buying it - SoftBits are very good (although slow) and the quality is amazing.
I bought that marine vinyl for my doors and it wasn't cheap, so you're unlikely to save yourself too much...
EDIT: ok, it seems to be much cheaper than I remember, although I cannot actually find the same stuff I bought...however, I'd still get it made for you. Unlike changing brake discs, for example, a roof does require some skill and it can look VERY bad if you don't have that skill.