Your best bet is eBay France! They go for around 15 € on there, and a lot will post to the UK. Make sure you search through
Enchères & Achat Immédiat (Auctions and Buy it Now) though and not Petites Annonces as that is small ads.
Orange France do a Pay As You Go SIM where you get free unlimited internet for 3 € per day.
On SFR it costs 0.50 € (for up to 2 MB) for every internet session you use, so if for example you go online, then come offline and go back on five minutes later, you'll be charged twice. This is what I used when I went over in October - but in hindsight I'd say that the Orange offer is the best, however, you have to add the 3 € bundle onto the account online at
www.orange.fr when you have the SIM which will require a knowledge of French.
Otherwise, you can go into a Orange/SFR/Bouygues shop when you're over there and buy one, but they're usually 30 €!

Also, be sure to take ID (passport/driving licence) otherwise they won't sell you one - it's the law there.
And it is correct that credit on French SIMs definitely expires. The more you top up though, the longer it is valid.