Buy a Technet Plus Single User
subscription for $599 USD. 10 keys for every edition of Windows 7 (plus a whole bunch of other Microsoft software). Overall, TPSU should be cheaper than buying individual boxed copies of the OS, and that subscription level will ship product DVD's to you. Make sure that you pick the right one though, as the Technet Plus Direct subscription is identical, but does not ship out product DVD's.
... and no, the product keys you get through Technet do not expire if your Technet subscription lapses. All that happens is you simply lose access to the keys, the downloads, and no more DVD's get sent to you until (or if) you renew.
Edit: Microsoft did originally offer a family pack for Windows 7 Home Premium which contained 3 Home Premium keys, but I don't believe it is available anymore.