You asked for an inexpensive, lockable glass display case.
Admittedly my source is family, but you may have a connection you
My husband had a convenience store. Various vendors would provide a
countertop show case when he bought the product. Unless he continued
stocking that product, he would eventually have an empty show case
to discard. He brought several lockable cases home for me to use.
Configurations vary - some will not work, and the cases tend to be
heavy, especially if they are made with glass. Plexiglass or plastic
might be better for transporting around.
Make friends with a local convenience store owner/manager by
becoming a frequent customer. Keep your eyes peeled for a case that
would meet your need. Ask if it would be available when it’s empty.
Also check with sporting goods stores - they have knife cases that
lock and might work. I hate to see something perfectly good be tossed
into the trash.
Good luck and think “re-purpose”
Judy in Kansas, who is gearing up for a weekend show and hoping the
weather continues to be this nice!!