I would think any stock for a current production model 1895g would fit. Some people upgrade their walnut stocks. I bet there are some out there. You might try a few WTB posts here and there. Good luck.
Just stating the obvious, but if you switch to walnut from an 1895G, you're going to have to change out your lever and bottom metal to compensate for the straight grip buttstock. Western Gun Parts in Canada sells a lot of stock sets and conversion parts. They post a bunch of their stuff on Gunbroker. Do a gun parts search, and type in "Marlin 1895". Their prices don't seem too terribly bad for what you are getting.