Ok, just checked. John can, at this time, make a scroll with true strike. At next level he can take the feat craft wand, but this would take a full day to make this item. His old teacher might have one for sale. or perhaps a larger city.
As a scroll it would cost 25 gp and some xp. but it could be done in a short time. It is a one shot item that as an object of UMD would require a dc of 21 [20 + caster level ]
scratch that. for you to use a spell from a scroll you have to first decipher script that needs a concentration of 1 minute.
by the way, the spell I am talking about here in on page 296 of the players handbook.
Ok, now I see the only way you would efficiently use this spell would be a wand, or a potion [50 gp/dose]. A ring is really expensive.