Try local Craigslist, Facebook Marketplace, local yard sales & thrift stores to avoid shipping. Etsy has them too & maybe you can work on the shipping costs with the seller, but sometimes they're cheaper sometimes not.
Some big stores do sell them you just have to check closer to the date.
And there's always Amazon Prime. You can find plenty of blow molds sellers on Amazon. If you hit the right one &/or Amazon fulfills it & you've got Prime you get zero shipping.