Since the Tanaka ones don't carry gas in the stock and/or pistol grip, I would assume that anything that fits the Maruzen CA870 or M870/1100 (if you cut out the gas tube) would probably fit on the Tanaka.
For example, some Clarence Lai customs on redwolf:
http://www.redwolfairsoft.com/redwol...l?prodID=22069
http://www.redwolfairsoft.com/redwol...l?prodID=22187
They're both Tanaka base, but appear to have CA870 stocks mounted. Failing that, real steel stocks may be compatible.
Here's my old thread on the M1100. I'm not sure if it'll help, but it may give you some idea of the compatibility of parts.
http://www.airsoftcanada.com/showthread.php?t=34585