Is the company you're working for British-based or French-based?
Actually, either way you should be covered under the French NHS system. This means that 80% of the cost of any medical treatment you require will be covered. If you're working for a British-based company then you need an EHIC card.
The remaining 20% can be covered by travel insurance if you're working for a British company. I would suggest trying Fogg, who do a lot of seasonal worker insurance packages. If you're working for a French company, then you would need a "Mutuelle" (personal health-insurance top-up).
I'd also rather strongly suggest that the company you're working for should be sorting this out for you!