Is there an icon set containing icons for different platforms/stores (e.g. App Store, Play, Windows 8 Store, etc.)

I can’t find anything about Win 8, but for Google and Apple you have to use their logos.

For Android / Google:

We encourage you to use the Android and Google Play brands with your
Android app promotional materials. You can use the icons and other
assets on this page provided that you follow the guidelines described
below.
You may use the Google Play Store icon, but you may not modify it.

As mentioned above, when referring to the Google Play Store app in
copy, use the full name: “Google Play Store.” However, when labeling
the Google Play Store icon directly, it’s OK to use “Play Store” alone
to accurately reflect the icon label as it appears on a device.

And the same applies to Apple:

An App Store badge must be used in both marketing and advertising
communications, such as TV spots, print ads, video trailers, email,
newsletters, and websites, whenever you promote your app offered on
the App Store. Only the badges shown here are approved by Apple.
Badges must be used as provided and cannot be modified.