Stealing chance is mainly by level, to the point it almost doesn't matter what item you need to steal. Apparently it's level 6020, assuming you're stealing from behin and have max lovers.
https://gamefaqs.gamespot.com/ps2/930240-disgaea-2-cursed-memories/faqs/44919

Your Thief should be around Level 6000 with 25 Thief Lovers and using a Hand with (preferably) over 100 Hit. I have regularly seen Bandit's Hands in the Item Shop with at least two decent Marksman Specialists that meet or exceed 100 Hit (and have even got one to appear with 125 Hit).

http://disgaea.gamexploiter.com/calcs/d2_item_steal.php

Originally posted by Kraleck:

Your Thief should be around Level 6000 with 25 Thief Lovers and using a Hand with (preferably) over 100 Hit. I have regularly seen Bandit's Hands in the Item Shop with at least two decent Marksman Specialists that meet or exceed 100 Hit (and have even got one to appear with 125 Hit).

I threw a couple marksmen I had already subdued onto a legendary hand I had by chance so it has a hit of 500 now. But still only 1% chance. I knew level was the most important factor in stealing but didn't realize to such a major degree.

Well for the record, for anyone who stumbles across this thread in years to come, my thief suddenly had a 99% chance to steal the Yoshitsuna at level ~7000 with only 16 lovers and any level hand.

Originally posted by Kraleck:

Your Thief should be around Level 6000 with 25 Thief Lovers and using a Hand with (preferably) over 100 Hit. I have regularly seen Bandit's Hands in the Item Shop with at least two decent Marksman Specialists that meet or exceed 100 Hit (and have even got one to appear with 125 Hit).

Bandit Hands in the shop occasionally get up to over 150. I've gotten a couple like that already.