I have successfully used the RBMicro Astatic D-104 mic element replacement kit, as well as an element that is available from Mouser Electronics. The Mouser link is:
http://www.mouser.com/ProductDetail/Kobitone/25LM024/?qs=XEjAOFiNPmWngviyFeX2Hw%3d%3d The element from Mouser doesn't include pre-cut foam inserts as does the RBMicro kit, but I have gotten very complimentary audio reports while using it in both SSB and AM modes of operation on a variety of transmitters and tranceivers.
73s,
Richard (WA4FUU)