I think this is a struggle regardless of practice model. Even for ordinary specialists...I only just found an allergist I really like for my patients and I've been hunting since years before DPC!
Some of the specialists I "inherited" from older docs at my old job and then just kept when I started my DPC. Others I used to like and are now on my "bad list" because they were completely obnoxious (for example, one subspecialist I used to refer to a lot had done a few things to show he thought PCPs were stupid but then he actually wrote that he adjusted a medication outside his specialty "because I am clearly the only one who knows anything about it who is seeing her"....I was floored and have never referred to him again.)
I have built up my network in various ways...perhaps I knew them from when I was in med school, perhaps I started referring at my old job, perhaps I had a new patient come who just LOVED their whatever-ologist.
The way I finally found an allergist was to start randomly sending different patients to different allergists who seemed like they might be good, telling the patient that I wanted a full report of what they thought, until we found one. I'm curious how other people do this. I've heard of "networking events" but honestly, all my favorite specialists are too busy living their lives when they are off work to be doing those.
My criteria for a good specialist is one who is good to the patient (competent, thorough, respectful, can schedule them in a reasonable amount of time, provides excellent care) and who is good to me (never treats me like I'm stupid, sends me a note afterwards, doesn't make a bunch of referrals to other specialists for things I can do myself, answers my questions if I call, etc). That can be surprisingly hard to find!
As far as expense, I know a handful of places that provide good sliding scale options or a payment plan but for some, I have to tell patients that it's what insurance is for (for example, I nearly always refer to Children's for peds). I used to feel guilty about that but the reality is that it's not my job to solve 100% of the problems with the healthcare system. I'm doing my part but there's not much I can do about the cost of the brain surgery one of my patients needed recently! And reproductive endocrinology is just always an investment.
Most of my patients are "underinsured" (have high deductibles) and a lot of those insurances will let me do referrals even though I'm not in network. If they have an insurance that requires an in-network PCP, I coach the patients on what to ask for and provide them with a packet of results and a letter from me to the new PCP. Sometimes they will switch but usually they'll have both of us and the in-network PCP is just there for referrals etc.
I'm curious how other people handle subspecialty needs when they comes up?