Sorry, I don't have any leads on vans. However, I remember that the airport has a useful website; I'd suggest looking there. There is also a bus from the airport to Lourdes. But with a large group, that might be almost as expensive as a van.
Do give yourself some quiet time in Lourdes. It's a remarkably holy place. The Masses in the grotto in the VERY early morning are really quiet and beautiful; however, if you decide to go very early, do identify the gate that's open all night--it's not the main gate. Also, the high Stations of the Cross, which takes you up a mountain overlooking the grotto, is remarkably beautiful. You can find schedules and details on the website of the shrine, and also at the information building as you enter.
Have a wonderful trip.