Some dive boats and dive shops do have a pony bottle requirement, and their requirements differ from shop to shop. One local shop for example requires a pony for all ocean dives deeper than 80 feet.
Using a pony bottle is a great idea, and I would not let this shy you away from a certain operator, in fact they may be the better choice because they are playing it safe.
Most dive shops rent a pony rig for about $12/day or so. It would include a full cylinder, a mounting bracket and a regulator with pressure gauge.
I would also make sure you are clear with the dive shop that this is your first ocean dive, and see how they react. Some shops won't allow you to dive without a guide or some training first, while others may have no problem. I know some shops provide a dive guide for the entire group too. You may have to pay extra for this too.
I would also try to find out where you will be diving. Some locations are just better for newer divers and some may be way too difficult for some divers.
I think the best thing is to get on the horn with all of the dive shops in that area, and see what they say.
Sounds like it is going to be a great time. Keep us posted on what you find out. I will also keep my eyes out for more info.
Ace