Here are some interesting costume ideas from some of the ones I like.
Fullmetal Alchemist:
Sadly, I couldn't find any place where you can buy a suit of armour like Al's. I guess that's not surprising though, really. It would be insanely heavy and expensive!
Black Butler
Dressing as Sebastian would be pretty easy--just look for a nice black suit, and don't forget the gloves. You could carry a teapot or flourish a fan of forks and knives.
For Ciel the darndest things come up when you search for costumes! It seems that for some reason, more people want to dress as Ciel wearing his girl disguise than Ciel in his normal Victorian boy's clothing. Who knows. This seller seems to have done a great job in making one of the dresses though.
If you'd like to be Ciel in his regular clothing though, there are several options. This is probably my favourite but if you are looking for the others, they are on the same site.
Oddly enough, I found an Undertaker costume on Etsy. I'm not sure how you'd manage to sharpen your teeth appropriately though.
If you wanted to dress as your favorite over-the-top, completely mad, man-chasing redheaded undertaker, yep, there's a costume for that too. It is in men's sizes but I'm thinking it will look more appropriate for the character if a female wears it. Will you choose a chainsaw or dual-wield scissors?
If you are a woman who wants to dress as a female there's always the screechy, clumsy maid, Meirin, who has a secret talent. Make sure that your glasses are easier to see through than hers are though, or you really will have a scary Halloween!
Spice and Wolf
I think if I ever go to a convention and am not dressed as something from Tolkien or the Whoniverse, I will probably be dressed as Horo/Holo. It really wouldn't be that hard!
Peeking from the bottom of the cloak, you'd need this.
Wear a little pouch containing wheat around your neck and carry an apple!
Mushi Shi
Really, I think only guys will be able to pull off the Ginko look. The clothes would be very easy--he usually wears a white long-sleeved button down and khakis. Wear a backpack and white wig. Ginko is usually smoking, so you could carry a hand-rolled cigar or cigarette (not sure which it is), but I think it's best not to encourage smoking. That one is your call.
I have a horrible feeling that I'm leaving out some great costumes, but I'm going to have to stop there. I realize that anime costumes are usually reserved for conventions,but why not wear them for Halloween?
