Minky
E-bay route and scratchbuilding is the way to go. Invest in some good craft tools and make up a recycling system of bins of odds and sods of plastic, metal and old toys. It is amazing what I have in my shed. And almost half of it would have been thrown away as junk.
Everything from drinking straws and coffee stirers to broken up electrical appliances for gears, cogs and weird shaped bits of plastic. Computers with heaps of tiny motors that would have been junked because they are out of date. Great for making models animated.
Basic packaging for day to day goods has so much throw away plastic (clear or otherwise) that you could salvage for raw plastic parts to scratch build shapes. Some are so abundant that you could make up duplicate models easily from your imagination.
One thing about ski-fi - you will find it very hard for the "nut and bolt counters" to say that model was never produced in that colour scheme or with that weapons system. You can prototype your own fleet of tanks without fear of falling foul of the history checkers that tell you when and where that mark IV scud tank hunter, chassis number az hge200938547, was built and what the welder had for his lunch on the day it left the factory in Crieff.
Good luck