Where can I find the C/W rating for PI48, PI49 and PI51 heat sinks?
I think you're approaching this the wrong way. Here's one way of going about it...


First, decide the "thermal resistance" (in C/W) you're looking for. (Note - values are usually given for unobstructed still air. Obstruction will increase thermal resistance, airflow will reduce it)
Then, go to a competent supplier such as Farnell (there are others) and look at their range of heatsinks.
Enter minimum and maximum thermal resistance values in the search menu ... such as 5C/W and 10C/W ... 213 results found.
Each heatsink has its manufacturer and full data listed, including dimensions and thermal resistance, so you know what you're getting. ("Multicomp" is an own-brand where this supplier doesn't want to reveal the source, or maybe they buy from several sources).
If you find an exact match for your own supplier's part there, now you at least have better specifications for it, and can buy it from whichever supplier you prefer.
If you'd rather go direct to the manufacturer, now at least you know who they are.