Where to get Kindle Fire HD apk files
Downloading and installing apk to Kindle
APK files are the default application files for Kindie Fire HD and these are files designed for installation on Android devices.
The default and safest place for getting apk files for your Kindle fire is simply from the Amazon app store and this you can get by going to apps > Store from your Kindle home screen. Once you've located the type of app you want either a game or not, you can start downloading it form Amazon by clicking on download which can be for free or paid.
There are lots of free as well as premium apk files available on the Amazon store and there are also other places to get your apk files besides the store place. One place I also use is the opera apps store at apps.opera.com. You would need to visit the opera apps site using your Kindle silk browser. Once you visit the site, the app store would recognize your device as an Android and provide you with a list of apk files that are suited for your Kindle.
If you are installing apk files from third party sites like opera.com, you should also know that Amazon does not approve of it and by default, they've disabled it on your device so you have to enable third party apps installation by going to your settings > Device > Allow third party installation.
You can either browse for apps or apk files on your Kindle on the store or simply search for it it you know the file name. One of the apps I've download and installed for free is that of Angry birds and it really went smoothly.
It doesn't take too long to download and install an apk file but you can always look at the file details before trying out a download to estimate how long it would take.
Downloading an apk file from the internet to a Kindle fire hd requires a Wireless internet connection and you may also have to buy it if it's not free or simply look for free apks if you want.