There is walk-in lab in our nearest bigger town where you can get tests done. Sometimes it’s a bit of a wait tho’, but if you are paying for yourself then no prescription needed. You do have to know what tests you want of course! Preferably in French.
I used to use this service before we moved over permanently, as I have to have regular tests. The basic (full blood count, inflammation markers and liver enzymes) was about 6€ for the nurse and 60€ for the tests.