Yes, Lidl mobile runs on the O2 network and SIMs are sold only at Lidl supermarkets. Officially you can only recharge with Lidl recharge vouchers sold at their supermarkets, but actually you can also use Fonic vouchers, which are sold at many retailers and gasstations, as well.
Additional information can be found at:
http://prepaid-wireless-internet-acc...id=6-aQmLmrozk (English, mostly written by me)
LIDL MOBILE ? Prepaid-wiki (German)
Allthough I've sold my spare Lidl SIM to VladS I could still pick up another one tomorrow since I will buy two more for Vlad anyway, and send it including insured shipping to Canada and PayPal fees to you for € 9.25. The only benefit I would have is € 5 of bonus credit if you provided my number upon activation.
Note that the promotional price of € 5 including € 10 of initial credit (+ €5 bonus credit if you fill in another Lidl customer's number upon activation) ends on September 10th. Afterwards SIM cards will cost € 10 again.
So if you take my offer you'll save only € 0.75, but you would have an active SIM card upon arrival (no need to search a Lidl supermarket and to wait up to 24h for activation) + an active data option if desired (may also take up to 24h for activation) and you would know your number before departure, so you could tell it to whoever may want to reach you and also record it to your voice mailbox.