Online hosted virtual machines : https://www.microsoft.com/systemcenter/configurationmanager/en/us/virtual-labs.aspx
Downloadable VHD of with sccm 2007 R2 installed. https://www.microsoft.com/downloads/details.aspx?familyid=E0FADAB7-0620-481D-A8B6-070001727C56&displaylang=en