Heroclix
One of my first main source of cheap minis were the Heroclix minis. Heroclix started making DC and Marvel themed minis, but they also include Star Wars and other settings.
They traditionnally come into blind boosters, with varying degrees of rarity, but you can also buy them online individually. Many websites let you do this, and then it’s mostly a matter of finding the one that will have the best shipping cost for you. As I live in France, the shipping cost is sometimes bigger than the cost of the minis themselves.
That being said, you can usually find interesting minis for less than 1€. Then it’s just a matter of repainting them. Heroes like The Thing or the Human Torch would make perfect Earth and Fire Elementals. Repainting The Hulk with a more natural flesh tone would make a great Ogre.
The whole game is then to find how you could reskin an existing figurine into a new one. The sculpt quality is usually quite good, and those minis are perfect for using the Dip method. The Dip will show many details that were hidden by the poor default paintjob.