When you're documenting a service procurement using this justification, the FAR limits you to two options: (a) follow-on procurements; and (b) white paper work. Many companies mix "expert services" and "only one source available" into an uncomfortable bouillabaisse that looks hinky and smells bad.
Here's the argument - they're an expert, and they know stuff we don't, and the customer likes them, so they're the only supplier qualified.
Sound familiar?