Personally i think it is all about the relationships you build. CPA's aren't going to give you any referral unless you establish credibility and rapport. In my small area it wasn't until i met lots of people and got warm intros till things started clicking on relationship based referrals. If you are just emailing random CPAs in a large metro area I'm not sure how much that will help. However if you are treating them like an Owner, creating repeat touch points, and creating rapport, I think they can be a great source of leads in a long tail proprietary search.

CPAs can be great leads. I'd prioritize in person outreach outside of busy season (as in during the summer). They are a ton of accounting focused conferences where you can find people with relationship with owners.