I can't answer every question you'll ever have about latex, but I can give you a start.
Making clothing out of latex is different than fabric because you'll have to glue it, not sew it. Sewing would make loooots of tiny holes along the seams, and latex is basically heavy weight balloon material. Also latex stretches quite a bit in all directions and has no grain lines. It's usually used for skintight things because of this, though cloaks, kimono, etc are made sometimes too.
It's been years since I've looked for sources, but Radical Rubber and 4D/Supatex were the main ones then. There may be other sources now, but probably not many as it's a pain to produce.
I'm not familiar with the cloak you're thinking of outside of a quick google image search. 0.6 mm is what's generally used for catsuits because it's a good combination of stretch and strength, you might want to look at 0.8 for the extra weight. (Yes, 0.2 mm makes a significant difference.) I don't know what glue we used for the seams, it was like an industrial-weight rubber cement. You'll also want to use rotary cutters instead of scissors.
Hate to break it to you, but yeah, this is gonna be really expensive. Both in materials themselves, as well as little things like rotary cutters and polish. Personally I would recommend a pvc/vinyl fabric instead, but maybe all this doesn't daunt you and instead piques your interest. I can't teach you how to make things over the internet (I'm a pretty crap teacher in general) but there are tutorials out there that can get you started.