I have a mid 2014 mbp; I bought it in June 2015 so it went with Yosemite on, but I've read that the mid 2014 is compatible with OS X 10.9, is it true? If so, how can I get it?
It is compatible. However, if you did not download Mavericks on another computer back in the day you will not be able to get it again. Mavericks is no longer offered as a download from Apple, so unless it's on your Apple ID purchase history you cannot get it. There may be some other sites that offer it but it's not legal IIRC.