We've rented the double from them twice now. I bought a universal rain cover and an organizer/drink holder thing for the handle bar from Babies R Us (online).
The rain cover works pretty well. It doesn't come down quite far enough in the front, so if it's really raining hard and the wind is blowing the kids still get kinda wet. What we've done in that situation is unbutton it a little from the top and let it hang down lower on them. This leaves the back of the stroller a little more exposed, though.
The organizer is nice. I really like it. The cup holders part of it hold water bottles really well, and fountain drink cups fit okay if they're not too large. Honestly at Disney we didn't tote our soda cups with us too long after our meals. But we usually had water bottles somewhere in the stroller or backpack. In my organizer I also store a few snacks, a couple of diapers, extra wipes, tissues, travel size sunscreen, travel size first aid kit, sunglasses, and germ x. I leave this organizer attached when I fold the stroller. Nothing falls out because it's all in zippered or velcroed pockets. (I do take any drinks out of the holders as that part isn't zippered.)
Another place I've bought stroller accessories from is
www.carryyou.com. I like this site because they tell you which brands/models of strollers each of their items fit on. They are a little more expensive, though.