Your two best bets for now (along with a lot of patience) would be Gunbroker and Ebay, which has once again allowed the sale of slides and other formerly "forbidden" parts.
I just did a check on Sarco's website and they have slides for $40.00, but they don't specify if they are .380 or .32. I have a feeling that the slides are identical, as they are unmarked and the calibre is determined by a separate component, i.e. the breechblock. Here's the link: http://e-sarcoinc.com/remingtonm51slide.aspx
(BTW: Your "contact us" page isn't working.)