VMware license can only be purchased for the current version, which is 8.0 at this time.
However, once purchased, and registered in your VMware Customer Connect Portal account, you can downgrade license keys to earlier versions directly in the portal, i.e. v8 -> v7 -> v6.
From how I understand you needs, the least expensive offering "vSphere Essentials Kit" might be to version to go for.
You can purchase licenses either from resellers, or directly on the VMware online shop at https://store.vmware.com/
André
PS: I assume that you are aware that vSphere 6 went out of support last year.