Tetranitrocarbazole is quite easily made from nitration of carbazole with ~95% nitric acid. Carbazole itself is, IMO, hard to synthesise (Carbazole wiki). But Merck sells 250g of carbazole for 25€, which is pretty convenient.
Fedoroff's Encyclopedia of Explosives and Related Items, Volume II, pp C48 - C50, gives quite some info...
I wanted to add these pages to my post, but I didn't see a way of attaching something (only links), how strange.
If you haven't got that book (but apparently you should have ), I could still mail them...