Good resources to validate product with paid ads?

Hey all, I'm building Dime (getdime.co), the easiest way to create and send professional-looking invoices for freelancers.

I'm explicitly not looking to validate the idea. I already know freelancers need to send invoices, and there are dozens of tools out there that do this. Instead I'm looking for advice on how to validate the product / solution.

My assumption is that running paid ads will be the quickest way to see if people understand the product and will use it. I want to specifically target first-time freelancers, i.e the people that don't know how to send an invoice yet.

Does anyone have any good resources, preferably by other indie makers who have run paid ads to validate their product?

Or does anyone else have ideas on how to validate the solution quickly? I'm currently already looking at Reddit and am posting there, but very hard to do self-promotion there.

Thanks!