Step 1: Bluestacks 101
Bluestacks is pretty great. Installs like any other Windows app. Try playing Draw Something using a mouse... lifechanger.
Once installed, you need to install an Android app that will allow you to backup other Apps. I chose ES File Explorer because it can do this and connect back to the computer to transfer those backed up apps to the desktop. There are other apps you can use and if you choose to do so, write us up an instructable doin' it your way. :)
Follow your nose and use the pictures here as a reference. Once you get past the third screen here, it's like installing an Android app on anything else.
Step 2: Let's Play Find My App
Step 1: Click search and type in the name of the app you're searching for... then click Find.
Step 2: Click install next to your app and it will take you to a list of markets you can download it from.
Step 3: Since this particular instructable is about using the program to get apps you can't find anywhere but google play, click the one in the Google Play column to proceed to Play and install the app.
Step 3: Now the Fun Part...
Click select then click the app you want to back up.
Then click Backup.
Once it's done, click back and you should have a shiny new folder on your imaginary sd card called... wait for it... backups!
Step 4: Copy Copy Copy Copy Copy
Click Select again and click the file you want to copy... AND THEN CLICK COPY!
Then look towards the top left corner and locate where it says Local, click this and then Lan...
(Side note: Next step only works if you have your computer setup so you can share to a folder on it from your network. If you don't know how to do that... maybe a later instructable... until then, GOOGLEFU.)
Click Menu, then New and Scan for you computer. Should be the only one that pops up.
Locate the folder on your computer where you want to save the app and copy it there...