Where to buy prepaid mobile phone SIM cards

As the title suggests I am looking for a German SIM card for my phone and ideally would like to know the address of the local T-Mobile "Handy" store. I am in Heidelberg today but will be back in Stuttgart (STR) tomorrow. Also what time do they close??

Thanks in advance folks!

Just FYI, I bought a blau card at Saturn on Konigstrasse for 20 Euro that gives me the card and 20 Euro worth of calls at 9 cents a minute. Can be refilled pretty much anywhere or on-line. They also sell a world card to call or roam anywhere for 20 Euro but the rates are higher for Germany at 19 cents a minute.

Thanks Adrianlondon for the info!

Hope that helps some folks!

Xtra-Card from T-Mobile is a oportunity, but there are some cheaper operators on the market. You can buy the cards of them online or in the stores of Aldi, Lidl, Penny or Plus. You find a list of prepaid discounters at http://www.clever-telefonieren.de/prepaid