How to fix bar stools that are too short
Um, not really. Unless you got the stools from a mix &amp; match furniture store like Ikea where you can buy the legs separately from the seat.

There are approximately 3 standard chair/table height combinations:

Table height (e.g. a standard dinning table; seat height ~18 inches, surface height ~30 inches)
Counter height (e.g. a kitchen counter; seat height ~24 inches, surface height ~36 inches)
Bar height (e.g. a bar or "high-top" tables; seat height ~30 inches, surface height ~42 inches)

Sounds like you have "counter height" stools with a "bar height" surface.