This is an old topic but I'd like to bump it as I haven;t found anything that matches it. Given that Foundation is only for OEM systems (you buy it with the OS), this should be the easiest R2 OS to get working. Thanks!
From pkeyconfig.xrm-ms - there're reserved two groups of oem:slp keys for Server 2008 R2 Foundation edition: ActConfigId {0e0d125d-173f-4752-8c1b-ced62118977f} RefGroupId 168 EditionId ServerWinFoundation ProductDescription Server 2008 R2 Foundation OEM:SLP ProductKeyType OEM:SLP RefActConfigId {0e0d125d-173f-4752-8c1b-ced62118977f} PartNumber X15-53994 EulaType OEM Start 1573000 End 1577999 ActConfigId {2b1690e6-1066-4b17-883f-4f759db1a7dd} RefGroupId 168 EditionId ServerWinFoundation ProductDescription Server 2008 R2 Foundation OEM:SLP ProductKeyType OEM:SLP RefActConfigId {2b1690e6-1066-4b17-883f-4f759db1a7dd} PartNumber X15-59689 EulaType OEM Start 1583000 End 1585499 So you can see there are supposedly 5000 possible slp keys in first group and 2500 in second.