They are gonna be hard to get cheap unless they are used..They are not being made anymore..Why i have no clue..
Depending on what kind of power you are gonna make 50's or 55's will work also.
I don't know why the 39#'s wouldn't work fine for me, so I think I'll check on those (I seem to think they were in the low to mid $200 range).
I saw that the MRT site is still listing them for $300, but when I called they had them for the new $500 price. Modular Depot (T&J) site has them listed for $330 but when I called I was told they would be $375.