1. There are a few on ebay that will work. Try to get one that's more like Ciel's length. I bought a SHORT short black wig for my first attempt and it was terrible. I recently bought a black Ciel wig and am hoping that it'll work better. I haven't tried it out yet, but I've played with it a little and it looks like it'll be great.
2. I bought my suit jacket at a thrift store. It's a little tight on me, but it actually fits my arms, which never happens. You might poke around a few local thrift stores to see if they have any jackets that might fit.
3. I have trouble with shirts too. I often choose shirts that are too big and then pin them in the back until they fit. Since there are a lot of layers to William, this should work for you. I'm not sure if it would work to do the same to a vest.
My William cosplay, including all parts of the suit (pants, tie, jacket, ect), glasses, stuff to work on his weapon prop, and both wigs I ended up buying, has only cost me about $90. The shirt was the most expensive aspect; I bought it new for about $30.
Gasukan July: Sega Genesis gijinka and Sage - Ronin Warriors
Gasukan August: Hakkai - Saiyuki
Gasukan September: Shazan - Gestalt
Gasukan October: Yakyu Jubeh - Battlefield Baseball