I will be using H&R 15mm spacers front and back, which require 60mm bolts. I'm having trouble finding locking extended lug bolts to go with them. Will it be OK to use 4 proper extended lug bolts with 1 regular locking bolt on each wheel? What's the standard practice for locking wheels with spacers?

 

Wouldn't recommend using the shorter locking bolt. If you can't find a longer locking bolt, try using a threaded stud of proper length and then you can use a locking lug nut.

 

If someone wants your wheels, they are going to get them. Consider your safety first.

 

Exactly, run the correct length bolts. You don't want to crash your car because of it, or end up breaking something else.