Agencies come in all sizes, so it may be possible to engage one even on a limited budget.  It all depends, of course, on just how limited.  If you're in a small media market and wish to advertise locally, you may find an agency that will take you on for as little as about $100,000 or so a year.  But in most cities, you'll find ad agencies want clients with budgets considerably larger -- preferably in the millions.  So your best bet is to find a creative team made up of a copywriter and designer who work together but without the whole agency infrastructure.  Look for people who have worked in major agencies and have considerable experience in the marketing communications tactics you plan to use.  Then, if you want to place media, you can go with a small media buying company.  You may find companies that specialize in media buying in your town that will take on clients with limited budgets.  Just understand that they are compensated based on a commission of 15% or less, so the media buy has to be large enough to make it worth their while.

The best way to find great creative people is to look for locally produced advertising work you admire and contact the advertisers to see who created the materials.  If you're looking for a copy and design team, consider contacting your local full-service printer and see who they recommend.  Other sources would be your local Ad Club or Art Directors Guild as well as Women in Communications.