They are available at AK and the waterparks. When we went last year, the waterpark mug was an insulated mug and was like $11 or $12, but the refills are free. At AK, it is a plastic green cup (that sweats) and has a huge animal head with a built in straw for a top. I think it was like $6 or $7, but then you paid a bit for the refill. SeaWorld also had a refillable cup, like the ones at AK (but without the giant animal head).
Obviously those toppers really bothered me. Totally changed the center of gravity on the cup and it kept tipping over in the cup holder. Very annoying and sticky!
Hope that helps.