(Am, is, are, was, were, will, have, has, had, may, might, can, could, shall, should, must, ought to, would)
has a contraction form with "not" (although some are fairly colloquial, or considered out of date):
(amn't, isn't, aren't, wasn't, weren't, willn't, haven't, hasn't, hadn't, mayn't, can't, couldn't, shalln't, shouldn't, mustn't, oughtn't to, wouldn't)
Amn't is neither standard nor historically supported; the word naturally developed into the modern word ain't, which is stigmatized.
Standard English essentially requires "I'm not" instead. Where avoiding the inflectional form is impossible, the standard awkwardly provides first-person aren't.
> willn't
Not sure what you were thinking here; there is a negative form of will, but it's won't.
> shalln't
And this is shan't.
If there is such a thing as Standard English, then it would allow phrases like “amn’t”, according to this definition anyway, as a casual style or register.
Evidently, it is usable to all speakers - you can come up with it yourself for the first time ever, and still be perfectly understood by every other speaker who hears it for the first time.
There is no One True Standard.
Also, you can abbreviate "it is" as "it's" and "is not" as "isn't", but what do you abbreviate "it is not" as?
But hey - if Stargate uses it (and they do - S09E04) then I'm willing to count it :)