Southwest Strings sells Sensicore Supersensitive Octave violin strings for $43.48/set. That's a good price, I think I paid almost $70 several years ago. You will have to set up your violin with enlarged grooves in the nut and the bridge for the larger strings, and switch the D and G strings on the tuning pegs so the very large diameter G string doesn't bend down at too great an angle to the peg. I let my luthier do that rather than botch it myself.
It's fun to play an octave fiddle, and it always gets a surprise reaction from listeners who are not aware that a violin might sound like a cello.