you could try itch.io, gamedev market, google search is your best bet, or you could commision an artist to do some for you. 

The reason you find few of them is there doesn't seem to be much call for them, or there isn't the monetary value to do them from other artists. Because most developers or hobbiests only do platformers and rpg type of games, thats where the 'bread and butter' is. :)

That said, maybe if there was 2D scrolling beat em up sprites we would see more of them on the game markets and encourage others to do create them.

Perhaps now you have asked and highlighted missing material from OGA, then the community may add them here in the future. I'm going to do some, but they will be more 'double dragon' style(8bit), and i wont be dong them at least until November this year.

Maybe leave a request here of what you need and someone may do them, and if your willing to pay for them, then make an offer for a commision. :)

Why not hire craftpix himself? https://opengameart.org/users/craftpixnet-2d-game-assets Drop him a PM and see if he'd do a request/commission?

"Wonder if some indie studios accept to sell thier old game sprites?"

Can't say i ever heard of something like that, it depends on the assets themselfs, whether they were commisioned for the game and the dev paided for an exclusive license to use the assets done by the artist, or paid for them out right, so depending on the license will possible determine what the dev can actually do with the assets, or if the dev owns the rights to the work then maybe its possible for them to 're-sell' them to you for use. depends on the agreements been made between both partys.

It wont hurt to ask i suppose, not sure how that will pan out.

Why not search here for an artist you like the work of, and PM them asking if they would be interested in doing the work for you, if you let them know what sort of budget you have, or create a thread seeking. You may find you get a better response then trying to get a response from messaging all the indie devs you ask to use their assets. Be descriptive, show screen shots of what you want the work to look like,pixel/vector etc, what actions you need, a little bit about your project. The more we know the more we can decide if we can help you, and beleive it or not, if the project sounds interesting enough you may find an artist that may work with you on the project and work for kudos/credit or another agreement. it's a long shot but it does happen. :)

Good luck, il keep my eye out and let you know if i pass anything. :)