I'm going to give advice here on the assumption that you won't be taking on a CTO. I think it's a legit choice, finding the right partner is hard. Anyhow, it'll be easier to find the right person after there's something tangible to show.
The first thing to clarify to yourself is how close your SaaS is to a CRUD app. The less actual processing of data in the background there is, the easier it'll be for you to get this app built.
The second step is to wireframe your product properly, including behavior. For this you don't need a developer at all, but a designer. I would strongly recommend looking around on Upwork. Upwork gets a lot of flak on HN, and in my experience, wrongly so. At any rate, since it's your SaaS, not the designer's, you'll end up designing most of it yourself. The designer is there to execute, so that you end up with a decent mock up.
Working on getting to a mockup will force you to get very clear on the exact functionality of your future app. For this reason, don't try to design everything. Design for the absolute bare minimum product you could see yourself using.
Having a mock up is brilliant: not only can you show it to prospects, but you can start to get quotes for the work from different developers. Because they're all seeing the same functionality, you can compare quotes better than if they each come up with their own solutions, design, view on how the app should look and act.
Get at least five quotes, preferably more. Agencies will cost your more than freelancers, and will generally pad their hours or fees. The end result with agencies can literally be worse than freelancers. There can be a huge disparity between quotes (up to 10X difference, I found). Part of it is is trickle-down VC money: agencies that got used to catering VC-funded companies will charge more.
I know none of this seems to talk about the development part, but that's because ideally, by the time you find the right freelance developer, they should be able to just take your mockup and get it done, the way a WordPress designer can take a design and get it done.
Again, all of this assumes no fancy algorithmic or data processing in the background, and also that this isn't a data-intense application.