Always got mine at the Icehouse in Freeport when I was over that way. May not be there anymore but they have Cypress Seafood and Cattle Company. Also, Nick's on the north shore of the bay at Basin Bayou. He lists himself as being in Freeport.
Then there is Sexton's in Destin. Always a good place but the Yuppies have probably run the price up there.
Remember, with Oysters, it's not always about price but more about quality.
Not sure about Destin, but for comparison I've purchased 100 lbs. from Joe Patti recently at $28 for a 25 lb. box. Texas oysters, and anyone that tells you everything is bigger in Texas is lying or hasn't seen their oysters. Tasty, but lots of work for small oysters. I also priced Maria's and they were $38 for a 25 lb. box of La. oysters. Either way, they've about priced them off my menu. Ridiculous!!! Sorry for the derail, hope you have better luck in Destin.