Hmmm...
A 26" barrel on a Ruger.....
If a 24" barrel doesn't give you the ballistics you want then the extra two inches is only going to make a difference way past 600 meters... Which means that you might want to reconsider your preferences.
But if you really want a 26" barrel on a Ruger I'm pretty sure you're stuck with finding a gunsmith.
Jimro