When making custom simple paper minis, there is one key website that proves to be the greatest resource: Google Image Search
My campaign I did was a near future earth where mechs dominate the battlefield and have changed warfare. The players are part of a United Nations task force (this allows for a mixture of different nationalities for the players, so they don't feel restricted to one country). One of the major influences for this setting is the video game Front Mission. I like their mech designs so i used them as base. This campaign isn't a Front Mission campaign, but it worked for the look and feel.
So a simple search of "front mission wanzer" revealed a host of white-background mechs. Perfect for making minis. When you are thinking of a theme of mechs you wish to use, try not searching the game or title, but searching the actual names of the mechs. In Front Mission they are called wanzers (pronounced 'vanzer' - german for wanderpanzer, or walking tank).
Other great search examples:
All of those provide many excellent workable pictures to slap on standees.
Next I used a couple simple steps in Microsoft Publisher to make the standees. These steps can be duplicated in other programs, like photoshop or even powerpoint. Although they may be called something different, and there may be easier ways to do it, but this is how I did mine.
I made little 1 inch square templates. It consisted of 2 squares stacked on top of each other. 1 inch wide, and 1.5 inch tall. Then 2 smaller squares on the top and bottom. These get glued together to make the bottoms.
Next I copied the shrunken image flipped it horizontally and placed it in the box above. This would make a mirror of the mech when the standee is folded together.
I didn't want both sides to be the exact same, I wanted to be able to easily tell the back from front. So I made the backs of the standees a solid black silhouette.
That way when I darkened it to complete black (using the less brightness button), the whole box wouldn't black out, just the mech.