Reinstalling software on a brand new (non-formatted) drive - MacBook
If you live near an Apple Store - they will often assist you in re-loading the OS that came with that machine since the license for that software transfers with the ownership of the mac. They won't care about a non-Apple drive so long as it works.

If not, you will need to locate the appropriate media. Some ideas are buying it from something like amazon or craigs list, ordering replacement media from AppleCare. Lastly, you might find a local tech that would help you, but you will probably end up paying for all of these in one way or another.

You might have to do some research to see what the correct OS for your mac is so you can determine which media will work.