I would encourage you to go take a look at Shapways for farm equipment and tires. Shapways prints 3D models that customers have drawn up. Look at farm equipment in all scales because you can ask any of the shops if they would print it in HO scale. This is very easy for them most of the time to change scales within their drawings. There maybe the issue that it is too small for HO but if you look at some of the Z scale items I doubt it. Over the past year there has been a lot more models added and the site is getting easier to search than it used to be.
Pay attention to what material it is printed with and the fact that a lot of the models do not show a built model just the 3D rendering. Once you see a built model you will understand. "Smooth fine detail plastic" is smooth and "natural versatile plastic" has a sand paper texture but costs less. You can see the difference between some of the pictures. It is your choice how to have it printed most of the time. Pay attention to what scale you are looking at as scales larger than HO scale will cost less and N and Z will cost more to have it printed in HO.
Just do a search using Farm Equipment to start with. You will then go through builders pages to see what they have. There are some things from Europe that look interesting but need translation. There are some models that are listed as HO.
If you are scratch building the amount of tractor and implement tires that are out there is huge now. There are even a few motors now being printed like flat head V8, V6 and small diesels.
I wish that someone would do a 12hp Briggs and Stratton that I need for the Mayrath grain auger I want to build. The next thing would be a plain old 15" tire and rim. There was lots of equipment that used this type of tire. It was common to put 2nd hand questionable tires on certain things.
The pieces that I am most interested in and looking at are the thrashing machine, Ford 8N snow tractor, the bale elevator if it can be done in HO, some horses with harness, birds, some interesting older Gleaner combines, NH side delivery rake and combine headers.
I wish I could draw 3D because there are some other smaller things that I would like. I now have access to a laser cutter so there are some things like my diamond harrows and cabs for the JD 4010 tractors that I will get done using that. Yes I am cheating with the 40 series tractors for my era by a year.
Here are some links and by no means this is all of them but it will give you an idea and a start.
I started molding and casting 10 years ago. I did this out of need for parts that I had built or made modifications to so that I only had to...
At rest in the yard waiting for the next assignment. This is a picture of my True Lines 4-8-4 U-2-g 6226 Northern. I bought this lo...
Van Hobbies CNR U-1-f Mountain. This model came from the last run and has a coaster drive, factory paint, raised numbers on running board...