If you can't get it from a local Plastics distributor, or after checking a local sign shop, try www.harborsales.com that's where I buy my 2 mil from. 4 mil can come from the same place OR if you don't mind a little printing, you can get it from the local gas station dumpster in the form of cola or cigeratte advertisement material.

Go to the SPAD links page and look for the Suppliers links to find the downspout locater. Usually can get the local building supply store to order it for you if they don't have it in stock.