The two in Font I've used are quite expensive, but the last couple of times, I've been to one in Moret, just opposite the F1 hotel, which was massively cheaper, and quiet enough both times that I just went as a walk-in, they saw me pretty much straight away.
Here is the location on google maps, you can actually see the clinic just about on street view -
https://goo.gl/maps/qlSR9Of the two vets in Font (sorry don't have the addresses now as I'm not going to use them again!), the one that has a horse stables, and weird opening times is a bit cheaper than the other one (though the other one is open more hours). I'm pretty sure that this is the expensive one with good opening times -
https://www.google.co.uk/maps/@48.407573,2.689541,3a,75y,172.51h,87.45t/data=!3m4!1e1!3m2!1sN8vxC0YVutHGJs7wnGpOdQ!2e0!6m1!1e1.
It's a tapeworm wormer tablet that you need before returning. The check-up is optional but most vets do it anyway. The Moret one gives you a free check-up and fills that part out too.
Make sure that the vet writes the exact time of day as well as the date, as I saw someone getting turned away because the time was missing (even though the date meant they were within the time limit regardless).