You are ready to contact an art consultant when you have a good-sized body of work of consistent quality and style. It’s not so different from approaching a gallery.
Art consultants are in larger cities for the most part. You can find them in your yellow pages under Art / Art Dealers / Art Galleries / etc. (They’re listed under Art Galleries in the Denver phone book.) As with everything in your career, start with the ones closest to you before expanding into other markets. Keep in mind, too, that many one-person firms might not advertise in the yellow pages, so you need to be on the lookout for them in newspapers and on the Internet. (Art consultants are often written up in local business news.)
Before you approach art consultants, find out about their clients and their requirements for artist submissions, just as you would do before you sent your stuff off to a gallery.
I cannot attest to the usefulness of them, but have found art consultant mailing lists from Caroll Michels and from ArtNetwork. You can also build your own, qualified list of consultants.
Approach an art consultant as you would a gallery. Start with printed images and/or postcards. Include a resume and statement if they help explain your work. A larger firm would have someone on staff to wade through various submissions before they get to the owner or partners, so your work has to get past several stages of vetting. If they like what they see, they might ask you for slides or visit your website. If they find something they think would work for a client, they will want to see the work in person. It’s a process that takes time to match up your work with clients’ tastes.