I'm not sure where you could purchase a dictionary for it. But basically you just save it and install it (configure, make, make install).
Specifically, download the dict-gcide_0.48.1.tar.gz
file, open and save it in your user's home file with the program Xarchiver, then with your terminal go into the created directory (which will be "dict-gcide-0.48.1") and run the following three commands:
root:# make install
The first two commands can be run as normal user, while the last command ("make install") is run as root. Then, open goldendict, press F3 (or select Edit, Dictionaries), press Add, enter the location of where you saved the directory dict-gcide-0.48.1, check the Recursive box, press Rescan Now, then Okay. At this point, it should work.