Being cheap/broke I am always looking for cheap if not free games that are not complete crap. After searching on steam for awhile I think I found a decent group of games with some possible speedrunning potential.6 games for $4.94 some of these games are on sale right now so it might cost a little more if you don't get them soon. I will be streaming these games over the next few days so come check them out!
Outrunner Hard Man Crazy Ball Adventures AironBall: The Floating Lands Trundle Deadly Traps
If you're on a budget, it might be worth looking into abandoned and freeware games. I'm sure there are plenty of undiscovered gems out there:
https://classicreload.com/a-z-index http://www.abandonia.com/en/game/all
And then there might be some people like me who are willing to give away 1 game at times under €10,- with the requirement of that people speedrun and/or race it against me/others.
Consider running Flash games. One of my favorites is the Fancy Pants Adventure series http://www.speedrun.com/fpa World 1 was OK, but worlds 2 and 3 are really good even for playing casually.
Web games are good if you're looking for free, quick runs. I would recommend Upgrade Complete!, but there are lots of good ones out there.