I too would like to know where people are finding plastic hex bases which don't have an indentation/lip.

The Reaper Minis ones that I have bought in the past have that outer lip/central indentation that mirrors the metal ones from IWM. The same is true of the Proxie Models ones and all the Litko bases I can find are the same width from top to bottom rather than sloped. These apparently no-name ones have a slot in the middle. The Corvus Belli bases are circular, as are the Games Workshop ones.

The hex bases in the box sets are sloped but also have no lip/center indentation. Does that kind of base exist as a separate product so that it will easily accept mechs that don't have built-in bases attached to their feet?

There are these 30mm bases from Reaper