I'd recommend doing it yourself using techniques commonly referred to as 'growth hacking'.

There are a lot of resources on the web to learn, but here is a very thorough and cheap intro: https://www.udemy.com/growth-hacking-masterclass-become-a-digital-marketing-ninja/learn/v4/overview (Sometimes it's only $15, but at the moment it's $30).

In summary, it involves using cheap or free tools and techniques to
1) Get visitors to your webpage (many different ways)
2) 'Activate' them (getting their email address and helping them take predefined actions with your product)
3) Retain them (getting them to be habitual users/customers)

If you'd like to discuss how to implement growth hacking in relation to your specific product, let me know,

best,

Lee

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.

If you want to get an agency to work with you for cheap, I'd use some social influence to help them understand the value exchange of working with you. Personal brand can go a long way when negotiating marketable value and developing relationships with other brand leaders. If you want to position your personal brand to appeal to an agency as a client, let's talk.