Heading to Coumbus & Dayton in two weeks

The widest selection of singles in the Dayton area in my experience is The Barrel House, with the Dorothy Lane Market in Springboro a somewhat distant second. The two Arrow Wine locations may have the biggest selection of craft beer overall and a decent selection of 375s/500s/750s sold individually but they're limited in terms of the amount of broken up singles they sell (whereas The Barrel House sells singles of everything they get and also offers 12 oz bottle fills of beers they have on tap).

I can't speak authoritatively about Columbus stores with singles. I recall The Beer Zone at Kenny Road Market offering lots of singles and an excellent selection overall, but it's been a number of years since I've been made it there. The Market District on 3rd has generally been the best beer store I've been to, but only offers singled out beers in the $11.99 or $12.99 build-your-own six-pack with the usual limited grocery store style selection of singles.

If you want some brewery recs as well, just in terms of quality I'd mention Branch and Bone and Little Fish in Dayton (and further out Yellow Springs Brewery), and Wolf's Ridge, Jackie O's (to-go only), and Columbus Brewing in Columbus.