The idler wheels have replaceable tapered roller bearing cups. It's not difficult to remove the old ones and install new ones.
By "axle shafts" I'm assuming you mean the spindles installed on the carriers that the idler wheels rotate around. Other than having some made up by a machine shop, and waiting for them. The only source I know of would be a Tucker dealer, or the factory. I'd be very surprised if Tucker didn't have them in stock. I don't know the current price, but I remember they weren't too terribly expensive. (Tucker prices on spindle bearings and seals were very reasonable.) In my experience Tucker ships parts promptly and they don't treat their shipping department as a "profit center"; meaning shipping costs are fair.
If you look at the way the spindles attach to the carriers you might think you could remove the one bolt holding the spindles in place and gently push the spindles out easily. I hate to be the bearer of bad news, but if your spindles haven't been removed from the carriers since they were assembled by the factory, you may be in for a real chore.
Last summer on the recommendation of Tucker's Jeff Godard, my snowcat buddy and I removed all the spindles on our Thundercat project to coat them with Never-Seez prior to reassembly. In our case we had 36 spindles to remove (1600 series carriers with damper wheels).
The carriers were off the machine and we had both a shop crane and a hydraulic press to make the job easier. Most of the spindles weren't too much trouble. But four or so were most uncooperative. The hydraulic press couldn't move the stuck spindles, and in fact bent the pieces of steel bar we used between the press and the spindle. My snowcat buddy is a very physically strong guy. I tease him about being "part man, part mule and mostly gorilla". But even with him swinging a large sledge hammer they wouldn't budge. I almost felt sorry for the spindles...almost! It took several cycles of heating with an acetylene torch and a lot of force to persuade them to come out.
I don't think it would have been possible to do this in the field, laying on your side on a tarp and swinging a BFH. I sincerely hope your spindles are more cooperative. Good Luck!