A "water hammer arrestor" ought to be really, really cheap. It is nothing more than a piece of capped pipe tee'd off to the side of the line in such a way as to contain a small air chamber that can absorb the shock of closing a valve in a flowing water line. For anyone needing one, buy the tee you need to install in whatever your water pipe is, an adapter to up the pipe diameter to 1 or 2 inches, a cap that size, and 2 feet of that size pipe. Should cost you less than $5 at Lowe's, HD, etc. Glue the cap on the pipe, glue on the tee, cut the line, and install with the pipe pointed up so the cap is on top. Bingo. Water hammer arrestor.