Find a Play Store APK
You can Google
play store apk or something similar, find a suitable (trustworthy!) source like this one and sideload the APK.
This does not require you to be rooted, or your bootloader to be unlocked, but it does require you to allow installation of apps from unknown sources (check your Security settings or equivalent)
Flash Google Apps
There are Google App zips for different Android versions you can flash. You can find these by Googling (again!) on reputable websites such as the CyanogenMod wiki page. Pick the correct version that corresponds to your version of Android and flash it over your existing ROM. This will install the entire suite of Google Apps depending on which version you flash. Micro for the minimal amount of apps and full zip for everything Google.
However, this requires you to be rooted and have a custom recovery installed, so the first option might be preferable.