what would be the easy and cheap source of some steel strips

[LCHIEN]

.150 to .180 thick. (approx 4 mm)

.250 to .265 wide (approx 6.3-6.7 mm)

a few inches long (at least 2 inches)

Probably needs to be steel and not aluminum. Mild steel OK or better.

[Black walnut]

Sounds like shim stock. Check with any well stocked industrial supply house. MSC, McMaster-carr, Grainger, or fastenal. You probably have a local independent as well.

[LCHIEN]

shim stock way to thin, way to expensive

[capncarl]

Probably in your file cabinet. Hanging file folders have steel strips about that size. Now if you have gone paperless, that’s a problem.