Yard Master & Research
To be cost effective a new flat bed is just over 20 on ebay. Backmann has two axle trucks for 16 plus shipping. You need 6 matching axles. Find an inexpensive lot somewhere on pieces that have axles. A yard sale find would do it. If find a set at least you will get an engine and extra transformer. Train show might have a parts guy and certainly used stock. Just some ideas. Locally you have on line marketplaces too. Ebay has generic wheel axel sets for 12. Best deal with extra for future finds.