Hi there
For Windows 7 you have to have your own copy --. If you have W7 on another computer and so long as it is a FULL RETAIL copy you can transfer the license to another computer in this case the "Virtual Machine".
The only other method AFAIK is if you qualify for the W8==>W7 downgrade then you can probably get a W7 "downgrade".
You could then after installing make an IMAGE and then restore to your VM -- this performs a P2V (Physical to Virtual Image).
After you've done this re-install W8 and it should work provided you haven't activated the W7 install after your "Downgrade".
So steps are:
1) Backup current W8.
2) Download and install W7 downgrade
3) boot W7 BUT DO NOT ACTIVATE
4) Image it with stand alone program like Acronis to external USB or .ISO
5) restore your W8 machine.
6) Create VM with wizard -- at prompt check "I will install the OS later".
7) Set the Boot device in the VM to boot from a bootable stand alone recovery so you can re-load the OS image created in 4)
8) Restore the OS using Universal Recovery -- this essentially does a P2V conversion.
9) Boot the VM -- you will probably get a BSOD for the first 1 or 2 attempts
10) Boot again but then use Recovery option -- the Windows recovery option should be your Windows downgrade install ISO or make a recovery disk / USB as per tutorials in the W7 Forum.
11) finally when the VM boots OK install VMWARE tools, and define what configuration you want BEFORE activating.
12) If VM does not activate use Phone activation -- just explain you've downgraded to W7 but machine fails to activate.
13) Job done -- enjoy your new W7 VM.
(Note only W8 PRO is eligible for Downgrade).
If you have to purchase W7 you might still find it available at some outlets -- don't buy a "Used copy" on Ebay etc.
If you have a TechNet subscription you can download W7 Pro / Ultimate / Enterprise depending on your level of subscription.
Cheers
jimbo