The South End of Delray Beach has a Shack that rents Boards and Kayaks. Also the last time I was in that area it looked like a new paddle board shop opened up next to Pizza Rustica, they may have some Boards for rent. Don't know much about the selection they will have but those are the only two places i can recall. If you head down to Deerfield Beach IWS should be able to help you out too.
I just did the search last weekend and I can tell you that Nomad, a little north of Delray in Briny Breezes, has the best selection (a number of real surfboards, not just foamies) and the best daily rate ($25/day I recall) in the area. Looks like there will be a big windy bump for you on Sunday.