The Shadows Devour You.
Perhaps Robotnik owns an iron ore mine and has the means to process said metals? He'd need things like silicon for computers and AI's of course.
Either that or he regularly loots cities and towns for them. It depends how established Robotnik is in the setting. If he controls his own area, then a source of metal may well be in his possession. As all his "subjects" are robotic, food and water only has to be acquired for one man, and he can have everything else built. If he is a roaming threat, however, then he would probably have to raid or steal for things. It depends on your villains resources.
The term "Great Man" is disturbingly interchangeable with "mass murderer" in history books.
Banned
He could also be very rich, inherited funds.
[[User Banned]]_ My Pm box ix still open though, I think?
Two-bit blockhead
Well, in the games, the goofy designs of the robots are justified by his immature personality. Dunno if it'd help you, though.
Weird in a Can (updated M-F)
Pretending to be human
Interesting topic!
I may end up spoiling my own work for this, but currently my own ideas are thus, as Robotnik first operates on Mobius and later on Earth:
On Mobius, I'd say he had taken over a high-tech corporation's technology and resources and either killed or somehow removed every top manager except himself, and maybe keeping the lower workers as slaves.
On Earth, he ends up working first for an anti-government terrorist group in a Third World country and manages to use some of their men to hold the family of a a defense contractor's CEO hostage to force him to "accidentally" let some of his weapons tech fall into the terrorists' hands. He then personally upgrades them and tries to start his own North Korea.
In my Sonic fanfic (a bizarre mix of the Archie one and the Movie), Robotnik (his design being the modern one) uses Badniks, silly-looking things (like in the game). He also makes use of bases called "zones" (e.g Star Light Zone, Chemical Plant Zone...) Now how can we Hand Wave his ridiculous designs (given the chance, my Robotnik would build a Badass Terminator-like robot) and his unlimited resources? Tails' Tornado was Hand Waved by him collecting scrap mechanics and metal (think "community-service" thing) to build it, but Robotnik wouldn't do that.