AK sells a refillable sports bottle ($1 per refill - I think I calculated break even at 3 drinks). MK, Epcot and DHS all sell souvenir mugs or sports bottles at different locations. (No refills - but MK filled ds's for cost of a small drink (holds a large drink).
I heard of people filling their resort mugs in the parks, but most of the cs places I can think of keep the soda fountains behind the counters so I do not know how they are filling the mugs up. Maybe I just don't pay attention.
Anyway, I do put my mug to good use in the parks and it does save us a little money: After a counter service lunch, you can poor the leftover coke into the mug and carry it with you (in the summer it stays cooler a lot longer).
Also if you grab a bottle of coke and get some complimentary ice from a cs you can save money by making the bottle last longer than a fountain drink.