Did you contact Micro-Trains? They don't sell direct but could give you the part number and a source that does. Also Walthers N&Z scale catalog lists trucks, it doesn't say if they have couplers or not but a phone call could find out ( 1-800-487-2467 ). BTW the hook type isn't a Micro-Trains product I don't think, sounds more like Marklin. Then again maybe someone switched them because they had mostly Marklin rolling stock but liked that particular caboose?
Micro-Trains does have an online store. It has been added since I looked at the website last. On the tool bar of their web site all the way over on the right roll over the box that says "Online Store". Have you considered cutting off the Marklin couplers off the trucks and body mounting Assembled #903 body mount coupler for Z & Nn3 cars (905)? I have done something similar with N scale freight cars and been happy with the results. I don't know about Z scale but Micro-Trains N scale caboose have body mounts from the factory. Here is a list of the Z scale trucks:
As Jon mentioned they may not make trucks for cabooses with the couplers on them. I know the N scale cabooses I have the trucks sit way back from the ends so even the trucks with extended couplers probably wouldn't reach. If you have a local hobby shop or club they could probably install the body mounted ones for a few bucks.
Micro-Trains seems to only make one length(extension) of truck mounted couplers in Z scale. Do you have any Micro-Trains freight cars that you could remove a truck off of and put on the caboose to see if the trucks with couplers will work? If the trucks off the freight car work then you could use 004 02 021 $6.75 Bettendorf Trucks, w/ short ext. coupler.