Church of Sinvention

Another site offering nice and robust looking leather (and vegan) collars.  These particularly took my eye because of the subtle addition of the hardware, instead of being big plates it looks smoothly riveted on and integrated almost:

Church of Sinvention

Corporal Dale

If you fancy some nice robust looking custom leather collars you could do worse than looking here, same site also offers paddles, slappers, restraints, cuffs, etc.etc.etc. in fact almost anything made out of leather:

Corporal Dale Leather Crafter

Fantasies in Leather

If you're looking for some interesting contoured collars (and others, including some nice flat silver ones) then you may wish to check out:

Fantasies in Leather