It is mostly steam for me, but I buy from other sources a lot more than "very occasionally." I've got accounts with a half dozen different DD services, and I use about half of those on a regular basis -- in particular GoG, Steam, and Gamestop PC Downloads (formerly Impulse) get a lot of use from me. I also buy (mostly older) titles used all the friggin' time. I grabbed a copy of The Witcher 2 for $4 in a thrift shop just a couple weeks ago, and I've got a shelf full of just the hard copies I've bought since 2010, most of which I bought used, although I did pick up a few new for about what I would pay used.
Basically, I go with whoever can give me a better deal, but I'll pay more for a hard copy than I will for a digital copy. I only buy digital in the first place because thanks to the sales, a new digital copy is frequently cheaper than a used hard copy[footnote]or even a rental, if PC games could be rented the way console games can[/footnote], and since these are PC games, used copies are hard to find anyway. If you go somewhere that sells them, you can probably find /a/ game you like, but it won't be the specific one you had in mind that day. Which makes a good find that much sweeter, but that's another story.