I have found a good source of free spring steel wire and use it almost religiously for my builds for personal consumption as well as my prototypes. The source is the spring steel loop that is used in the plastic. (after market), hub caps/wheel covers that seem to be littering the edges of the roads and highways. I just cut them, straighten them, and then bend them to the shape I require. It makes undercarriage and cabanes effectively free.
Another source is from old mattresses and old furniture which use steel springs. The same straightening etc applies.
When adding undercarriage to a tiny model as in the mini scout or mini speedster, old bicycle spokes are adequate but only if the wheels are not too heavy/large and the undercarriage length not too great. Spokes are somewhat resilient but not as good as true spring steel.