Tents are on sale in many of the Hypermarkets in France (L'Eclerc / Geant / Carrefore / Cora / Intermarche} ... and there's bound to be one within striking distance of wherever you are!
They only have a small range and I don't know if they're branded or not ... but when I've looked at them there has been quite a range of sizes available (not huge family tents though).
Most of the big supermarkets (Carrefour / Auchan / Geant etc) have a good selection of camping equipment - including tents. Prices are considerably cheaper than over here too.
Another good place to look is Decathlon - most cities have one !
We are planning to do a camping tour in France in August, and have to debate between taking our heavy (2x extra luggage costs on Ryanair), or a smaller tent. I had also considered buying one on French Ebay. Would that work for you? You could pick it up en route if you check the location carefully. Do let me know if you find reasonably priced tents in the supermarkets - I didn't have a lot of luck with their websites... good luck.
It may cost you a fortune to take extra luggage, now, on Ryanair. Our son came back from Girona, not a lot above the limit (only one, heavy bag - no tents, etc) and we had to pay £125 in extra luggage (over the phone, by credit card, as he had run out of money). Most of the weight was stuff he was bringing back for us! French supermarket tents can cost very little - and there are even makes you'll have heard of. We saw one selling Khyam at around half the price we paid in England. The prices at Decathlon (above link) seem expensive compared to those we looked at - however, I suppose these are 'pucca goods'.