255/60/15 alternative?

Yup, that's about it. 255 is the width in mm, so if you want the same width it has to be a 255. 60 is the sidewall ratio, ie, the sidewall height is 60% of the width. You can get the same height with a different width and ratio, but if you want the same width you need the same ratio. As for availability, yup, not much out there in 15's anymore besides BFG T/A hockey pucks and the slightly better (IMO) Cooper Cobra's.

Mickey Thompson sells Sportsman S/T radials in a 255/60/15. Their Sportsman S/R radials come in 26x10x15 or 28x10x15, so, either 1" shorter or 1" taller in the same width as 255/60/15 (27" tall and 10" wide). The do sell their ET Street S/S tires in a 27x10.5x15, those are a DOT approved street/strip tire. And the section width is listed at 10.1", so, it's really the same width as a 255. They also sell their ET street R in that size if you're really not worried about anything other than dry traction, those are also DOT legal but just barely. They would not be a good choice for cold or any kind of wet weather.