The answer here is probably not to go for an agency.

Find a freelancer or consultant with the neccessary skills to work alongside you.

If you have a marketing plan, note down your name against any tasks you can do yourself and commit to completing them. Everything you can't do - find friends, family, interns or freelancers to help with.

If you haven't got a marketing plan, find a freelancer to help you create one as your first step.