It's really difficult to buy trailer brakes without the parts in hand at a trailer store. There is a sticker on the inside of the backing plate once you pull the drums that, if legible, will tell you the part number for the entire assembly, which is absolutely the way to go- much easier than replacing shoes and hardware, and hardly any more expensive. Drums can be measured, but it's the same story- a good old fashioned brick and mortar store is what you need.
Plus you really should pull the drums first anyway so that you can determine if you need bearings, races, seals, etc.
Rob, Sarah, Ella, Ava
2017 Eagle HT 29.5BHDS -- 2011 F350 CCSB 4x4 PSD