Where can I get...

Any online metals supplier should have brass round bar. Search for "free machining brass", or "360 alloy".

McMaster has just about any kind of acorn nut you could want:
https://www.mcmaster.com/acorn-nuts/=353317d3b51d4d3682c8496e4c72660dkf1gawtx

As for the idlers, I'm not sure what ID the Baders use, but if it's a 1/2" bearing ID, I'd check out eBay. The best, IMO, are made by "vwjackstraw":
https://www.ebay.com/str/knifegrinderparts

Though OBM wheels are a little bit cheaper and pretty good as well, or there's several other sellers. Average cost for an aluminum idler seems to be about $20 to $30 these days.

 

Your amazon link is a tube of goo

I'd buy Bader Wheels from Stephen Bader Co
OEM always fits best.
Cheap chinese wheels create more tracking problems than they solve.

Tracking problems, try more belt tension first, pull the toolbar out.

"Stainless steel 3/16 in cap nut dome topped as is used in Randall knives"

I'd call that an acorn nut.
Fastenal if you have no other local options,

If it's a 1/4 inch thread, you still have to know the thread pitch.
In USA SAE threads there is a UNC or UNF thread

You won't find any references to a 3/16" thread unless it's 100 year old book.

If it's smaller, you use the number thread sizing
#10, #8 and both have coarse and fine.
There is a 12 but it's old phased out, rare and uncommon

Use the caliper to get diameter
https://www.shars.com/products/measuring/caliper/6-dial-caliper

Use the gage to find pitch
https://www.shars.com/4-84-screw-pitch-gage-1


download the Starrett thread chart
https://www.brokenbolt.com/images/starrett-inch-metric-tap-drill.pdf


If you're trying to tap that thread, don't buy hardware store taps, they are ALWAYS carbon steel not HSS, they are always hand taps, not machine taps, they are always plug and not taper taps.

Get a HSS spiral point tap, from a pro industrial supplier.
Get two, plus a bottoming tap


Also = go to a good hardware store, or industrial supplier
Get a screw and nut of every size and every pitch, label them well.
Now you have a fast eyeball reference you can practice on with the calipers and gauges. Metric too if you want, but that's pretty un american.

 

You know, I just found out the other day that in america at least, the inch is defined by the metric system? Kinda funny!

 

Officialy 1 inch = 25.4mm

 

Yup, we define all our English measurements by the international metric standards - meter, kilogram, Kelvin, etc.
The current standards are defined by physicists and make no sense to the average person - did you know that the Kelvin scale is now defined this way:
The kelvin is defined by setting the fixed numerical value of the Boltzmann constant k to 1.380649×10−23 J⋅K−1, (J = kg⋅m2⋅s−2), given the definition of the kilogram, the metre and the second.

The USA was all ready to switch in the 1970's, even passed it as a law. But, the education system was run by old teachers (average age was probably over 50 when I was a kid) who did not understand the metric system or physics. My eighth grade science teacher was born in the 1800's. It failed because no one was taught to use it. Eventually they dropped the issue. The other issue was economic, the entire manufacturing system would have to retool. All suppliers would have to get new measuring equipment to sell. The final blow was the recession. With price controls and raging inflation, there was no desire to upset things any more with a new measurement system. The only vestige of the law is that all consumer goods in the USA have to have the contents stated in both metric and English, which was the first phase of the change.


Even as a child in the ancient 1950's I could see that figuring out how may times 13/64th" goes into 33 1/3" was not the best way. It was no problem dividing 5.2mm into 847mm.
Can you imagine expressing wavelengths in feet and inches???

As a funny aside, it hasn't been that long since they stopped marking anvils in stone weight.

 

I don’t know if I would use this on a knife