Inner tube sizing has always seemed unnecessarily specific to me. Inner tubes are very stretchy. A standard "road" tube is usually labeled as 20-25c, but with just a few lbs of pressure it will balloon out to several inches if not inside a tire. When installed, the tube is contained by the tire (and rim bed), so it's not going to stretch out. A standard 20-25c road tube has zero problems filling out 28-32mm tires.

I suppose a road tube in a 2"+ MTB tire might be more of a problem, but I feel like even that would probably work OK.