The metal Thermos brand drink containers (FunTainers or something like that) are expensive, but so worth it! My DD has been taking water or milk in one for a couple years now. We've definitely gotten our money's worth out of it.
I agree 100% - spend $15 now and it'll last and save you $$ for at least a couple/few years.
Bought one for ds when he was in K last year (btw- I didn't realize I would want one until about October or November - and they are VERY hard to come by then - get it now while all the school supplies are out on the shelves) - and even if he didn't drink all his milk - it was still cool (not cold) when he got home - and this is with FULL DAY Kindergarten. THey really work!!!
DS's is still in perfect working condition (just be sure to keep up on rinsing them ASAP each time it is used) - so I just bought another one for dd, who is starting K in Sept. I figure with hers - I will rotate with milk and buying a big jug of apple juice and putting some of that in. Way less expensive than the boxes - you'll make your money back in a month!!
I know the price tag looks high - I thought so too when I got mine last year - but go for it and buy the metal thermos!