I asked a similar question and didn't get a definitive answer, but after some looking into it found that there is an "official" disney bus, its run by mears, so is the one in WDW actually I think. Anyways, at DLR, unlike WDW, its not reliable, apparently, or isn't all the time. I've actually used that twice in the past, and had no problem. What was suggested though by a travel guide was to go to the airport and just find the shuttle area, its clearly marked if I recall correctly, and get on the first shuttle that comes regardless of company, and pay directly there. I checked ahead on some of the companies listed in that book and they were all the same price to buy ahead as they were to buy once there, some offer round trip discounts. Prices ranged from about 13 - 18 dollars one way, disney bus was the cheapest, but can make a bunch of stops before your hotel, shuttles hold less people so they will likely make less stops.
This is a little unnerving as you don't have your shuttle tickets in hand before, but I remember seeing people pay on the shuttle I had tickets for last time I was there, so its done. I guess the big advantage is you dont have to wait for a certain company, just whatever is there.