Teflon washers for cart, were can I get these?

Teflon washers which you place between the mounting screws of the cart and head shell.

I've tried RC Hobby shops and either they were to thick, wrong diameter etc.

Kinda shocked that it's so hard to find.

Try McMaster-Carr. FWIW, I've always preferred the sound with metal washers.
Agreed regarding source of Teflon washers, and the superiority of stainless steel in that application. Most metal washers are stamped, and hence have a perfectly smooth side to them, which should face the headshell - the other side has a slight raised ridge which you can feel with your fingernail - this side faces up.
Now there are no absolutes, and I encourage you to try everything, but in general, Dan, Palasr and I have had the same results.

While you might be able to tame (or mask) some other problems in your analog rig, my bet is that you'll prefer brass or stainless. McMaster is a good source for a lifetime supply of all varieties.

Thom @ Galibier
Okay always learning all the time, obviously more to this than I thought. I never even gave any thought of using either stainless or brass washers instead.

Just wanted to protect the head shell from the screws.

I've gotta ask; if you are doing this then you are actually having to take the time to re-align the cart to hear the difference which also involves other changes because of the weight difference added on by the different washers.

What are you hearing different sonic wise?

My thought would be that your alignment and over all new set-up would never be able to be repeated 100 percent so the sound would vary anyways, in the end what are you actually hearing the different washers or new set-up over all.

Do you torque the screws in at a specific weight?

Albert Porter uses and sells a torque wrench calibrated for perfect torque specified @ 0.4nm maximum.

I would think that this would have an effect of some sort.

Albert says; he has tested for ten years, first with the Rega Torque Wrench and this last year with this specialty tool. Replacing inferior hardware and hitting proper torque has ALWAYS improved sound.

Albert says; I've had success with all the high cartridges listed below:

Air Tight PC1, Benz Glider, Benz Ruby, PC1 Supreme, Koetsu Coralstone, Koetsu Jade, Koetsu Rosewood, Dynavector XV1s, Shure V15VxMR, Allnic Puritas, Audio Technica AT150MLX, Ortofon A90 and others.

Albert goes on to say; Those who have not replaced their cartridge hardware with non magnetic, high quality stainless steel screws, torqued to perfect spec, you have not heard what your turntable is capable of.

Between the head shell and actucal cart;

Are you placing anything between? My thought is that there would be allot going on here. I read someone useing a carbon fibre mat "sheet" and was surprised over all with the results.
I replace one washer at a time. That way the cart doesn't move and no need to re-align, just check VTF. I use an allen wrench and go to "two finger" tight without strain. I want my cartridge to be rigidly mounted so that it becomes a single instrument with the tonearm. The idea is to allow the tonarm, base, turntable, etc., to handle the vibrations as a unit. Any material between the cart and arm that would affect how vibrations move into the arm will affect the sound.