Regarding sources for wheel spacers, I do not know of any off-the-shelf other than Unverferth which I agree are more expensive than they should be. Most any machine shop can design and fabricate a pair for a reasonable price.
Since the tractor uses wheel bolts versus studs and lug nuts, steel would be the prefered material since it will have to be tapped for the bolts. (Use some anti-sieze on all bolt threads and recheck torque frequently until they all stabilize.)
Regarding tires and
ballast: Having liquid ballast already is a step in the right direction. If you need the traction of the R-1 AG tires, then the spacers are the answer. While 4" per side is probably a good number, you could push it to 5 or 6" if you really had to.
If you could go with R-4 Industial tires, there are several potential advantages. First they give you a bit more overall width (about 2"), but more importantly they lower the tractor almost 2" and also will hold more fluid for even lower center of gravity and increased stability. You can still add wheel spacers with this combination if you still need to.
Since your tractor seems fairly new, you may be able to work a deal to swap your AG tires and wheels for Industrial. Transfer the fluid you have and add more for 75% fill and give it a try. You can also add cast wheel weights with this set-up. It should be much better, but if you still need to go more, then do spacers.
Regarding the Power Trac tractors, they do not have dealers so you have to do your own maintenance and repairs or farm it out to several shops. The diesel models use German Deutz engines which are used in a variety of agricultural and commercial equipment.
The rest is mostly "standard" hydraulic componants with some proprietary valves and cylinders. Again, most any hydraulic shop (of which there are many more than brand-specific tractor dealers) can service a PT tractor.
One of PTs mottos is "build it stout and keep it simple". They began and still are a mining equipment manufacture and I can tell you that not much else is abused more than mining machinery.
PTs customer support is both good and bad. On the one hand you can actually talk to people at the factory who really know the machines. On the other hand, they do not cover outside labor on repairs even under warranty. They take pains to hide the source of otherwise standard off-the-shelf componants just to make life harder so it seems.
I bought my 16 month old PT-1845 used in April, and have had no problems. For my extreme slope conditions, I could not modify my TC-30 to go where I needed to go, so I had to look at an entirely different type of machine. Your conditions may not be so extreme that some combination of the above suggestions may well resolve your problem.
