I've been trying to find a black 90 mm reach x 26.0 quill stem to fit a new road frame that I picked up, but quill stems are getting scarce. I finally found a nice Cinelli 1A for a good price on eBay and it's in very good condition. However, the seller mislabeled the stem, and it has a 26.4 mm clamp. My LBS is reluctant to install it as the handlebars rotate freely when it's clamped down and doesn't recommend using shims. However, I have noticed on some websites that they recommend shims to make 26.0 stems fit 25.4 handlebars. Is this considered safe? Or should I continue my search for a black 26.0 x 90 mm stem?

I found some shims to be reliable. I never knew they were there until I dismantle it for various reasons. - it never budged (shims to adapt a different size stem/handlebar interface, this have no issue with turning the bars without the wheel)

As for shimming a quill stem into a steerer tube... I never found more than 2 dimensions: french(22mm) and 22.2mm, I've heard about 21.15mm in the USA but never dealt with them (I'm from Europe). - this is somewhat tricky to shim, but my best bet would be to use a wedge type and not a cone-expander type if mismatched.

If you think about it for a minute, you'll realize that alloy bars first came out as
simple, unreinforced, bent alloy tubing in whatever shape you found preferable.

They then progressed to the sort that is much safer, with a steel reinforcing
sleeve in the center where the stem clamps it. In my own view of the forces
and stresses involved, a shim (properly sized and installed) ought to be no more
of a hazard in this position than anywhere else on the bike.

LBS's are understandably cautious on stuff like this. Whatever they can reasonably
charge for the installation is not worth the liability.

With regard to shims on the stem insertion, this was commonly done back in
the day, with shims cut from aluminum beer cans. It works fine, and was
the preferred fix for undersized, cheap assed, French 22.0 stems that often
showed up in low end bikes (Raleigh did this sometimes, among others) that
were stuck in 22.2 steerers and headsets. Just make certain your shim is
wide enough to support the entire length of whatever tubular thing you are
shimming out and you should be OK.

You can buy shims to take a handlebar from 25.4mm to 26mm, like this https://www.rivbike.com/product-p/hb91.htm

If it was my bike I'd do it.

I've used aluminum shims on aluminum and steel stems and handlebars without any problems.