Will any of the less expensive locking skewers fit older bikes? Like with 120 or 126mm rear spacing, and horizontal dropouts?

Are you referring to the skewers with actual locks in them, or just the bolt-on style that you need the correct hex-key wrench?

Just about any skewer with threads can be shortened with the correct die and a hacksaw, been there done that. The main thing with using new skewers with older horizontal dropouts is clamping power (external cam < internal cam) and the face serration of the both the nut and the clamp end. Modern skewers for vertical drops need less clamping power (external cam) and many have slipperier interfaces, as that clamping force is not needed.

When you're looking for your skewers, it may be a good idea to look for some made for fixed-gear applications (if marketed that way), as they should have better clamping power - and the appropriate length.

That's kind of what I was wondering -- the clamping force. Had a wheel with an external skewer on a bike with horizontal dropouts, and it came out while riding it. Also wonder if I might have to run the threads farther up the skewer. Some for fixed gear would probably be what I need.

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...the problem with most of the ones made and sold is the material used in the rod portion. Very few locking of wrench type "security" skewers are made with steel rods, so the resulting threaded portions won't take the clamping forces required for horizontal drops .

Most of them say on the packaging somewhere to use them only for vertical drops.