Central Hobbies is the only retailer that I am aware still selling Hyde mounts and they seem to be selling out their existing stock as several of the Hyde mounts are no longer available. :-(
Merle Hydes contact information (email and phone) are still listed on Central's web site; so, hopefully if you need a specific mount you can still get it direct from the manufacturer.
I will continue to hoard my remaining stock of his excellent engine mounts - I have yet to wear one out and I started using them almost 30 years ago!