in the past they have been very specific about which motors to use. RS555 is more of a physical standard than a performance standard, so using a different part number motor that begins with RS555 could give you an advantage over another team, and is likely to be prohibited.
Of course you should ask on the Q&A to confirm this.
I have not seen any other source for the motors....is there one listed in the "where to get more" listing in section 10?
http://usfirst.org/roboticsprograms/...nt.aspx?id=452