For some reason, there aren't a lot of sales consultancies. It's more training firms that dominate - Huthwaite, Mercuri etc. It seems that clients expect - and prefer to hire - trainers to improve their sales side. Possibly because they see it more as a skills issue than a process or strategy issue.
Personally, I've done what "Mars A Day" suggests - I run my own firm specialised in Business Development coaching & consulting for professional service firms (consulting firms, lawyers, accountants, etc.). If you go down this route I'd suggest finding a tight niche - the general SME market is not well known for paying good rates.