I'd like to know more about your product to be able to answer this question most accurately, but here are some quick tips to guide you:

1.
Launch an affiliate program. You don't need marketing budget to scale an affiliate program, you would pay affiliates a % of sale or flat rate commission when they refer customers.

2.
After launching an affiliate program, you can negotiate deals with bloggers, online publishers to promote your offer. If you can get them to send out a dedicated e-blast promoting your product, this would be really effective.

Some questions:
What is your product?
Do you offer a free trial? Or some kind of discount for first time subscribers?