If you have an old router that can use 3rd party firmware, you should be able to do it with that.
That's what I would do personally, and have done. The Asus RT-N12/D1 routers (and maybe their RT-300 B1 routers, which seem like they may be an updated model with the same hardware, unknown to me if they still take 3rd-party firmware), will run FreshTomato or AdvancedTomato. AdvancedTomato is a slightly older Tomato core build, but has a more advanced HTML 5 UI on top of it. Hopefully, perhaps, AdvancedTomato will build off of the FreshTomato builds.
Both of those 3rd-party firmwares can be set up as Repeaters, and even the stock AsusWRT factory firmware will, I think, do that as well, on these particular models (just not as well).