Lowes seems to have them the cheapest. Just bought a couple more cases of 6' cords @ $1.49 each. They had a sale last year where they were 79 cents for a while, but that didn't happen this year. You may want to get a couple tubes of silicone dielectric grease and squirt a dab in those vampire plugs before you put the wire in and slide the cap on. It will definitely keep the water out, since that seems to be the likely source of trouble based on the description of your current setup.
I get tons of water being here in FL, but only had my GFCI trip once this year after a particularly hard rain. I noticed a section of the lights weren't on anymore, so just reset it and had no more problems. Heres a pic of a couple of LEs on highground. I use PVC legs with heavy duty velcro (harbor freight) to attach them to the sides of the cases. This keeps them off the ground and still lets me break them down and store easy. I would use longer legs to keep the cord ends off the ground as well if I had to place the case in a low spot that might get standing water.
Mike E.