My current favourite is cardboard since it "never runs out", can be obtained in large quantities for free and is super easy to work with. Toothpicks/cotton buds, skewers, matchsticks and popsicle sticks are all great sources of wood. For very thin details that would normally be easily damaged (such as handrails, window frames or Plarail-style couplings/coupling rods), I have a 3D pen that uses PLA filaments. Not really practical for large pieces due to how it works.
If you can't easily find plasticard, one of the cheapest alternatives could be signs for things like "No Parking", "Beware of Dog", etc.. If plant nurseries where you live are anything like here, they might sell a bunch of coloured plastic/resin flowerpots and planters you could cut into sheets and pieces if you have the right tools. I'm currently experimenting with nylon zip (cable) ties for custom track systems, but have yet to get the results I'm looking for.
If you can't easily find plasticard, one of the cheapest alternatives could be signs for things like "No Parking", "Beware of Dog", etc.. If plant nurseries where you live are anything like here, they might sell a bunch of coloured plastic/resin flowerpots and planters you could cut into sheets and pieces if you have the right tools. I'm currently experimenting with nylon zip (cable) ties for custom track systems, but have yet to get the results I'm looking for.
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