I think the model is useful if you’re unsure whether you’re ready to hire BDRs internally. Several years ago, I hired Vorisght (now Acquirent) to start a BDR (sales appointment booking) motion at a B2B software startup. It was helpful to start small (I think we had 2 outsourced BDRs for either 10 or 20 hours per week). It can be hard to know what good looks like if you hire one full-time employee, and it’s the first in that function.
To find a good vendor, I’d search online for “best sales appointment booking providers” or “best lead generation services” until you find a list with vendors that appeal to you, create a shortlist, and backchannel (see if you can connect with someone like you who’s had success first-hand).