Hey,
You can use the v3.4.0 release which supported solidity >=0.6.0 <0.8.0
The link for the contract: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.4.0/contracts/math/SafeMath.sol
The release:
You can use this to install the package:
npm install @openzeppelin/contracts@3.4.0