GOG is excellent for this reason and my experience playing older/unsupported games from them is top-notch. It stings to buy an older game again, but they're almost always inexpensive, and you can say, "Well, this is the very last time I'm going to purchase HoMM3" (or your game of choice).
They also do fine as a purchase platform for modern games, but it's less than perfect because modern games are supported and some smaller publishers choose not to push updates to their offerings on GOG.
They also do fine as a purchase platform for modern games, but it's less than perfect because modern games are supported and some smaller publishers choose not to push updates to their offerings on GOG.