I will be in germany for 3 months, Where can I buy a prepaid cell phone in frankfurt?

I'd recommend you buy a used, not SIM-locked mobile at home and just buy a prepaid SIM for it here in Germany. I'm both with Vodafone (I have a contract with them) and congstar (prepaid; that's basically T-Mobile), and both networks work pretty fine. If you'll be in a rather rural area, I'd recommend T-Mobile, as they have the best network coverage. Within cities, you might want to opt for Vodafone (known as D2) or even e-plus or o2 ("can do"). Those are the four mobile network providers in Germany who own the networks, but they have many resellers and brand names of their own. If you intend to have a German bank account (which is highly recommendable) I'd suggest you go with T-Mobile/congstar or D2/Vodafone, as you can recharge your prepaid phone from any ATM or via your homebanking system. National calls will be at 9 eurocents/min; fees for international calls may vary. And mind that in Europe, you don't pay for airtime; we're on a "caller pays all" scheme.

O2 Voda phone Eplus and t-Mobile are the most known users here in Germany ,but you can get prepaid cards at many local supermarkets with their own brand of tarrif and phones cost as little as 10 euros but for a good phone on offer go to a local electical store and you can get 1 for 150 euros with camera,bluetooth and everything you need on it,.

If you want something simple and cheap, I recommend going to a "Saturn" or "Media Markt" store. Sometimes they have great offers. For example you get a phone for 15 € and a 5 € prepaid-card incl..