a druid 5, planar shepherd 9 can do it as long as he choses a plane that those devils are native to (shavarath, bator etc...)
this will get you EX SU
and SP
of any outsider or elemental native to the chosen plane(within your ws HD limit)
if he had a level of hierophant, he can grant uses of his wildshape to others,
so if you could just find this guy and hire(force, bribe, enthrall etc...) him to give his wildshape uses to the party for the day... you're set
note: you are now able to ws into
any other form the planar shepherd, hierophant could wildshape into for that time period... (chosen by the hierophant when he gives the ability)
EDIT: just realized that devil is 3 HD... what level is the party at? ... and are you the DM?
...and what does the party look like, are we talking 7 optimized wizards here? or more like a monk, 2 rogues and a healer?