If you don’t see your size combo, you can just order duplicates instead. My car needs the 24/18 which is not offered here, so I can just order a 24/24 and an 18/18. But I won’t:
Some combinations are less expensive at the mothership, so be sure to check over there first. Also be sure to check different combinations; the 24/18 for my car is cheaper, but both the 24/24 and 18/18 are more expensive.