Surely they will be supplying everything needed. Even the earbuds.
If you have to supply your own earbuds, and this is a one-shot deal, I recommend just using a cheap set of buds, or just wear regular headphone, like Sony MDR7506's.
I actually have a similar gig situation coming up. I play bass for the Church of God district praise band, and next month the church we are playing at uses IEM's.
To make matters worse, it is a Behringer system. I plan on bringing my rig as usual, plus my FutureSonics buds, and a headphone extension cord in case the IEM's are not wireless. I might even carry my pedal board in case they don't want me to use my amp. I do this gig every two or three months at different churches, and
none of them every have a real sound tech. Can't wait.