I have a 72 RT2 that on its 3rd over piston.  Can it be bored any larger?  I haven't seen any listings on Ebay for pistons larger than 3rd over.   Where can I get pistons for a rebuild?  Should I replace the sleave?  If replacement is necessary, who will do the work?

I've read that going any larger is taking a chance - walls too thin.

I'd be tempted to look for another cylinder, on standard or first oversize.

Couple on ebay.com for a lot of money.
Yamaha list 4th over as part number 308-11638-01.
Not sure where you're based, so hard to recommend machine shops.
I've heard on the forum that Bill Bune is great if you're in the US, if you're in the UK the PJME are very good, rest of the world I've no idea.

I've had trouble with 2 of my RT3 360 Enduros that were on the 4th oversize (done by previous owners). The cylinder liners separated from the cylinder and caused an air leak resulting in a lean condition overrev. The shop that does my boring recommended not going past the second oversize because the liner will be too thin. The 360 liner is 4 mm thick when new as opposed to the 250 being 5 mm. Plus the 360 generates more heat.

I'm not aware of anyone resleeving these. Best solution is a NOS. There is one on eBay item # 195571672120. Looks crusty but would probably clean up with vapor honing and a mild polishing of the liner. There are some used ones that are worn out and some misidentified, I would not consider them.

I have a NOS cylinder, standard, first and second oversize pistons and rings. Send me a PM if you're interested.

Yamaha go to 4th over which is only one mm. Two of mine are out to two mm over 82mm & someone's is vintage racing one regularly since I sold it. Other I just belt around here on the land but that cylinder has done a lot of work on another guys VMX bike. Mind you that particular one is an RT2MX cylinder but essentially the same. Ist one I'm talking is straight RT2. No problem going to 4th. Don't worry. That's my experience.

This past spring i had a 360 cylinder with a cracked sleeve. I called LA sleeve which they used to make sleeves for these 360s and still can BUT as i recall it was going to cost $600 to do which for me wasn't worth it. I ended up selling it on eBay and finding 2 more at the swap meets this year.
You can also try Millennium technologies but i doubt the cost would be much different IF they can do this.

same problem with cylinder/piston from may DT 360:

imgur.com/a/McdsOCq

Any idea if there is another size available?

The original sleeve is quite thick in places, mainly around the intake and skirt area, the top above the exhaust not so thick.

I have just finished off 4 Re-sleeve jobs on the 360 this week, they are more difficult than most but can be done.

Thanks for all the help, everyone.