In all fairness, I haven't been to every bar in Billings.  For the ones I have visited I am unable to order a "real" whiskey sour.  When I say "real", I mean two things:

1) Don't use a sour mix

2) Must have egg white

It's the second one that is likely the issue for most bars, but a little raw egg never killed anybody...not many people anyway.

Here's what I like:

2 oz Bourbon (okay, so it's a bourbon sour)

1 oz simple syrup (equal parts water and sugar)

Juice of 1/2 a lemon (maybe .75 oz or so.. I don't measure that part)

1 Egg White

Throw that in a shaker with no ice and shake it until it gets nice and foamy from the egg white.  After that, add the ice.. shake it 'til it's cold, strain it 'til it's smooth and garnish with a cherry if you have one handy.

If you're not freaked out by bacteria or whatever, give this one a go.  You'll thank me later.

Oh, and if you know of a bar that makes this, let me know!