Just call around to local dirt n' gravel guys. First, get the lingo right. Gravel is stone mixed with sand - also called crushed bank - used for road beds and such. For drainage, you want crushed stone, which is just that. Everybody uses different names for the same stuff too, so watch out for that - tell the dirt guys what you are doing and ask for recommendations. In my area, crushed stone is somewhere in the $15-20/yard range (delivered). Most places include delivery when pricing, but some do not, so ask.
15-18 yards (20-24 tons) is a normal tri-axle dump load. Ask how much they can bring in one shot. Some places call it 20 yards, but it is really 18. Small 6-wheeled dumps (F750-800 size) generally carry 7 yards. Also, don't be surprised if you end up getting considerably more than you think you'll need - material doesn't go far. I made a little french drain this summer that was only about 100' long - it required a full 24 ton tri-axle load of crushed stone.
I brought in 35 18-yard tri-axles loaded with gravel, stone, and loam this summer. I used a place that was cheap, but charged for delivery by the mile - I paid a boatload for delivery (~$100/truck) but there material was so much cheaper than the close guy that it was still cheaper to pay for delivery.
JayC