I have a Yaesu FT-2800 in my truck.  It is held in the mounting bracket by four screws.  I, also, have a Realistic HTX100 in the truck.  This one is also held in its bracket by four screws.  These screws are set into knobs that can easily be put on, or taken off, by hand.  Where can I get some of these to replace the screws in my Yaesu mount?  I've done several searches without success.  I have sent a query to Yaesu but have not received a reply yet.  Any assistance would be greatly appreciated.

The most common sizes are 4mm, 5mm and 6mm mounting knobs. Truck stops carry these knobs in thier CB radio department. Try searching "Workman CB products". Just make sure they are not too long as they may come into contact with the P C board or other components. The owners manual may have a list of the contents that come with the rig and usually contains the specs for the mounting screws.
ETA–– link workman under "radio knobs and covers"

Workmans is definitely a start.  I've bookmarked the site.  Now, I just need to figure out the size of the screws I have.  There is no information on them in the owner's manual.

Thank you for your assistance and the link to Workmans.

McMaster-Carr

I bought (of the appropriate metric thread size) machine screws and wing nuts, ran the screws through the wingnuts with a touch of threadlocker and made "wingbolts".

Ewww, I like this idea.  A little adapting and improvising works for me.  Thanks for the idea.

For the Ft-2800M, the four mounting screws on the sides are M4 thread x 8mm (I just checked, screwed in M4 screws).  Do not use longer screws or they may damage something inside.

I purchased my replacment knobs at Lowes. After thinking about it I dumped all my knobs. I switched to hex head screws to add a level of security to the radio after seeing some equipment stolen from some work vehicles. The thieves just turned the knobs and removed the mobile radios. At least now you will need a tool to remove the radio.