I personally would avoid Sisal, when I was doing research for natural fiber rope that would be outdoors I learned sisal is made from cactus fibers, where manila is made from Abaca over in the Philippines. Manila will hold up to UV/wet a lot better than Sisal.
AFA where to get it, I got mine from a spool at the farm store. If you have any in your area, that's where I'd look first. It should be right next to the Jute/Sisal ropes, it'll just be labeled Manila.