Thank you for getting back to us.
It’s not possible to directly purchase 3.7, but in your case you can purchase 4.0 Standard Edition and I’ll manually activate the 3.7 license in your account (since 3.7 is freely upgradable to 4.0 Standard Edition). So if at some point you decide to move forward to Sitefinity 4.0 you’ll also have a license for it.
Please, keep me posted on how you’d like to proceed further and I’ll be happy to assist.
the Telerik team