A few days ago Kaeng Kong Productions sent me some information material about their newest offers concerning the SY99. So, they are still existing, but they have moved to another location. Here it is: 

Kaeng Kong Productions

MIDI MUSIC MARKETING
Thomas Klimisch
Belderberg 39
D-53347 Impekoven
Phone: 0049 228 644479
Fax: 0049 228 644811 

btw: They obviously want to offer an internal Hard-Disk-Drive (520 or 1000 MB) for the SY99 (price is about 1000 German D-Marks or lower) 

Hong-Kong 

Tom Lee Music (shops all around town) - bad sales service but large stock and sometimes very cheap prices (if you don't mind ex-demo and no warranty prices can be ridiculously cheap!) 

A very small shop called Haughtons which specialises mostly with EMU and Waldolf stuff. 

Tom Lee (make sure to go to the Tsim Tsa Tsui branch for information, then buy from the cheaper Wanchai Mariner's Club branch) if you want cheap DATs (how about USD$100 for a 2nd hand), check the electronic stores in MongKok. 
 

Musitronic

Somwhere, probably in Germany, there is (has been) a company called Musitronic which feature three kind of memory expansions:

First product is a double-the-internal-ROM expansion for the SY77 (TG-500) that also features four additional RAM memory banks for user voices.

Second product is the same as the first, but it was designed for the SY55 (TG55).

Third product is a sample RAM expansion for the SY99, which will add additional 5 MB of sample RAM to your synth.