There are painting straight edges available for about $7 apiece. Here's one I found on Amazon:
You might find them cheaper (and possibly smaller, when used for trim work/etc) at your local hardware store, and they generally don't come with ruled markings because they're intended to get covered in paint anyways. Additionally, unlike many of the other ...
If you have a budget for this then drafting triangles are a easy solution, and they give you physical examples of two common triangles (more useful in introductory trig or analytic goemetry then in pure Euclidian geometry, but still).
By design they have nice, crisp edges designed exactly to use for line drawing.
Aluminium carpet joining strips are another hardware store option. They make nice straight edges for use with craft knives and would also serve well here. They're a better shape than cheap metal bar stock.
Take a first step by forbidding straightedges also.
This will make them understand that every construction possible with straightedge and compass is also constructible with compass alone (Mohr-Mascheroni).
You can introduce with Napoleon's problem: Divide a circle in 4 equals arcs. First with straightedge and compass and show that the ruler method you ...
Stock metal comes to mind if you want something slightly better than plastic/paper type things - in the u.s for example this can be acquired at hardware stores and cut to size relatively easily, here is a link for reference: https://www.homedepot.com/p/Everbilt-3-4-in-x-48-in-Aluminum-Flat-Bar-with-1-8-in-Thick-801917/204273967
Take an ordinary piece of paper and fold it. This must be the simplest way to produce an edge that is absolutely straight. A folded paper can be used to verify the straightness of a ruler, which depends on manufacturing precision.
You would be showing how to improvise a useful tool from readily available materials and you might be able to teach some ...
A cheap Venetian blind with thin plastic slats can be cut up with ordinary scissors and will yield many straightedges of any length you choose. The slight bow in the blade will flatten out when pressed against paper, and makes it easier for fumble-fingered people like me to pick up the straightedge from a flat surface.
I have lots of ideas, which, it turns out, aren't going to fit into a comment.
If you don't need something too long, but want something that is pretty uniform and consistent, popsicle sticks (also sold as craft sticks) might work. The are pretty durable, but relatively cheap and, ultimately, disposable.
Others have suggested paint stirrers (which you ...
Try shopping for a "paint guide".
These products have:
a straight plastic or metal edge
a grippable portion that is not on the edge
no ruler-like markings
Drafting triangles also have unmarked edges that are intended to be drawn along, but their accurate angles are too useful in your intended application.