Hi Peter,
Eduard makes a replacement barrel (#34026) for the Tamiya SU-122. Here it is at Sprue Brothers for $7.50...
http://spruebrothers.zoovy.com/c=sh0er1dsmAtXStqt5ZcDSmK50/product/EDU34026AirConnection has been good to me. I've ordered a number of things from them. All arrived promptly and well packaged.
They have some other goodies for the SU-122. Here are some Aber PE sets...
http://www.airconnection.on.ca/ABERsu122.htmThere isn't a lot on the net about the SU-122. Here's a good page though. It's on Battlefield Russia...
http://www.battlefield.ru/index.php?option=com_content&task=view&id=60&Itemid=50&lang=enHere's another good page. It's in Russian, but that's okay since it's mostly pictures...
http://armor.kiev.ua/Tanks/WWII/SU122/Same with this page (it's kinda slow to load, so give it time)...
http://serkoff.narod.ru/photoalbum77.htmlFor books there isn't much there either. The largest collection of SU-122 pictures and drawings I've seen are in Ground Power, Jan. 2002 (1), Soviet Self-Propelled Guns (2). It's kinda rare though.
The magazine Modelist Constructor 1/2000 has quite a bit in it too...including the winter camo paint job which is white with little green trees painted around the edge. It's rare too. Here's a review...
http://modelingmadness.com/others/books/sadlermc12000.htmThere is a new book out by Polish Militaria, #240 Su-85/100/122. I haven't seen it so I don't know how much it does or doesn't have about the 122...
http://www.jjfpub.mb.ca/polish_militaria.htmBest of luck with it.
Karl