My ds was on Pediasure for 2 years and I agree that you can't buy it in the parks. I brought my own case box of the stuff in the car when we travelled---a week's worth of those things are heavy! And I was mixing it half and half with organic whole milk (for all his bottles). Made travel loads of fun, all that planning ahead.
Anyway, they do sell it at Wal-Mart, as well as Publix in FL. You could always fall back on that.
With the half milk/ half pediasure mixture, I would make about 2 qts at a time then pour back into the pediasure bottles and tighten the lids. Then freeze them. Worked great for long car trips or a day at the parks.
The best price I found on the single serve bottles was at BJ's Warehouse Club, if you have one near you it was worth the membership price just on the Pediasure.