If you've completed the W10 Upgrade, Microsoft has a feature in it which lets you rollback to W7 for free, as long as you didn't erase the ~Windows.old or other 3 ~Windows backup directories within 30 days of doing the Upgrade to W10.
If you are outside the 30 day timeframe or deleted any or all 3 of the Windows backup directories needed for the rollback, then you are out of luck
, unless you purchase W7 factory media as kemical mentioned or buy MS retail legit media on ebay or amazon.
Best of luck,
<<<BIGBEARJEDI>>>