I have seen the PVC water tanks sold somewhere but not sure where. The problem is that it may be custom made for the titanium/stainless heat exchanger. The thing is that with chillers that use titanium heat exchangers, the copper tubing that carries the freon is joined to the heat exchanger with compression fittings. This is where the most common souce of freon leaks in chillers occurs. It is possible that the fitting has just become loose and it could be an easy fix. I believe that the heat exchanger is sized to the chiller and that PVC tank is sized/shaped to contain the specific heat exchanger size/shape. They are different on different chillers, it's not a one size fits all thing.
Heres how I found a tech to do a check-up on my Chiller several years ago. I found out from the local supermarket who it was that services their lobster tank chiller. They used an outside company, some big chains use their own maint. department. Turned out to be a one man company and a real nice older guy who came to my house to check it out.
Don't know what else to tell you other than to try something similiar to what I did. Or find a company in the Yellow Pages who does commercial refrigeration for stuff like lobster tanks. If you look hard enough you will probably be able to find somebody willing to look at it. Better chances if you live in or near a large city.