where can I find adjustable handle knobs in metric thread?

Look here on McMaster-Carr. On the left you can select the metric thread size you need.

-- Rex

Another source is Reid supply.
http://www.reidsupply.com/

-- Michael :-{| Diapers and politicians both need to be changed often; and for the same reason.

Yet another source;

http://jwwinco.com/

I second McMaster-Carr. The nut-gripping part on one of these on the tool rest on my cheap Harbor Freight lathe wore out really fast, so I replaced all of them at once w/metal ones from McMaster. Granted, it’s just pot metal, but they still beat plastic.

-- The whole aim of practical politics is to keep the populace alarmed (and hence clamorous to be led to safety) by menacing it with an endless series of hobgoblins, all of them imaginary. - H.L. Mencken

I bought mine at WoodCraft. I bought the larger size, cut the thread off, re-tapped to my desired threading and used a piece of “all-thread” to complete these adjustable bolts. This was one of the first upgrades I did for my astronomical telescope mount, in order to eliminate the “Allen screw” syndrome that you mention:

Buy HERE

(Below) Tool-less RA & DEC Adjusting Clutch Levers and Thumbscrews – I found the adjustable levers at a WoodCraft Store but they only came in either 1/4-20 coarse thread (too small to drill out and re-tap) or 5/16” threads (got these because they allow for re-tapping) so a good friend tapped and drilled the 5/16” bolt heads on the lever to the required 1/4-28 fine thread pitch and added the all-thread. Spacers are required to miss the RA housing and should be 5/8” long but still leaving 1.5” of threaded bolt on the end.

-- HorizontalMike -- "Woodpeckers understand..."