Also, why work as a contractor if you already know what SaaS the company wants? And if you don't know what SaaS they want, how do you know you want to work for them to find out?
Even if that's not the case. If it's genuinely out of their price range to get built custom just for them, which it almost always will be, then they should be willing to collaborate just to get it at all.
Sorry, I guess you don't ever need to do any actual contracting at all if it's just exploratory. I was writing from experience as half of a "software development agency" so we did do custom work, I just noticed that a lot of what we were building could easily be turned in to a product.
I will say though that actually doing some work for them, basically getting carte blanche to poke around their workstations and getting rundowns from their staff on how to achieve their tasks, reveals a lot of opportunities. You come across things like them moving over to a different pc. "Oh we need to use this because it has our X on it, which we use to do Important Task. It's not made any more so we can't get a new version".