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/

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.

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

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..."