I appreciate the replies. Thank you for trying to help.
I sat in front of the computer and researched, and I finally came up with the answers.
The cylinder can be rebuilt, but Kubota does not offer a kit, because that would be intelligent and considerate. You have to sit down with a parts diagram and look at various web sources and make your own list. I will post it here, along with the current prices from Messick's. It comes to a little over $160.
My new question: do I really have to use Kubota seals? Generic seal kits for hydraulics are available for about 20% as much. No one is going to tell me Kubota makes its own seals. I know better than that. And there is no way I'll believe other companies that stay in business selling seal kits are putting out junk year after year. No one would ever go back to them if their products made cylinders fail prematurely.
I'm no engineer, but common sense tells me it should be possible to measure the rod and the piston and find a company that will sell me size-appropriate seals. Otherwise, everyone with an old tractor or backhoe would have to scrap it when the hydraulics leaked.
For other people with the same problem, I will post what I learned while looking for the parts. These parts will work with any tractor that uses the same cylinder. A bunch of Kubota tractors use it. I made a chart, but it probably won't look right on this forum. Sorry about that.
The steering cylinder itself is part TA040-37803, and some places are asking about $1300. That's obscene. It should be closer to $200, based on similar parts for other machines. I would like to support Kubota, but I'm not a charity for giant corporations. If I can get parts cheaper, I will. If I find a source for cheaper aftermarket seals online, I will post the info here.
Maybe some day I'll get ambitious and try to make my own Kubota steering cylinder from a generic.
Diagram MPN Type Needed Each Total
30 TA040-37570 piston gasket 1 16.80 16.80
40 04816-00450 O-ring 1 5.10 5.10
110 08511-02512 bush 2 6.14 12.28
120 04810-50500 O-ring 2 2.92 5.84
120 04817-50500 O-ring 2 4.42 8.84
130 TA040-37560 rod gasket 2 16.80 33.60
140 TA040-37580 seal 2 24.43 48.86
150 TA040-37880 cir-clip 2 17.15 34.30
Total cost: 165.62
EDIT: it looks like you don't have to buy new snap rings (circlips), so remove $34.30 from the cost, leaving a total of $131.32.