You can get Flaked Maize, I know my local has it in stock and can post it.
There are a couple of differences between brewing grade and cooking -
Brewing grade has the germ removed, reduces the oil and protein content.
Brewing grade is pregelatinized, doesn't need boiling or a protein/glucan rest, just add with the rest of the malt.
It isn't flaked like corn flakes (or even rolled oats) it a fine kibble and should be called Micronized.
Personally I think using a bit of toss-up between using Sugar or Maize grits, well up to say 20% of the total fermentable. Maize was widely used in UK breweries to lighten the body and colour of some beers (sort of gone out of fashion), in the US and Canada they use heaps (up to 40% of grits) to dilute the high protein content of 6-row barley (not an issue here).
What are you making?
Mark