When getting custom monitors made do most audiologists have access do different brands or do you go with what they have?
The first audiologist I went to (Beltone) didn't sell any brands of IEM, but would take the ear impressions and give them to me to send to the company that I was using. I used the first ones for a set of Westone UM-56 custom ear tips for their universal fit monitors, to replace the universal silicone tips that came with my Westone 3 about 12 years ago. But you can mail your impressions to whomever you want to buy from.
After that I started going to audio shows in 2009, like CanJam in LA and Rocky Mountain Audio Fest in Denver, and they would have someone at one of the booths taking ear impressions to send to a variety of companies.
So I would highly recommend doing some research, and pick a brand regardless of who your audiologist is affiliated with, and even better,
try to find an audio show to try out some universal fit demos. At these audio shows, most companies will have some universal fit demos to try, but I find that much of the time the demos are not quite as good as the final custom fit product which is better.
JH Audio almost always has some very good sounding universal fit demos, and the final custom version sounds even better. Westone has great custom IEM, but they don't make universal fit demos of the custom models, although they sell commercial universal fit IEM that sound similar enough to get an idea of what you'd be getting (their W60 and W80 are great, but the W80 need more power than W60 which can be driven easily by an iPhone lightning adapter or their bluetooth adapter cable).
As long as the demos you try don't have muddy bass or sibilant treble, then they are not likely to sound worse in the final custom fit model - except in one case, where my UE 11 Pro final version was quite bass heavy until I discovered that they needed a headphone output with a very low output impedance (like less than 1-2 ohms output impedance). From an old iPhone 3Gs the bass was boomy and poorly controlled, but from a 5.5G iPod video or a $1000 maxed out Woo WA6 amplifier they were great. But any amp with a 10 ohm output impedance on the headphone jack would make their bass lose control.
I got three sets of impressions done at CanJam 2009 in LA, and got a pair of Westone ES3X, Ultimate Ears UE11 Pro, and Jerry Harvey Audio JH 13 Pro. I was able to hand deliver my personal ear impressions to the different companies right there at the show. Now CanJam is held at Rocky Mountain Audio Fest each year, with 1 or 2 shows a year being held elsewhere like in NY or London, and they have at least 15-20 companies peddling custom molded IEM with demos.
I did a huge detailed "Flagship IEM comparison review" on Head-Fi.org after CanJam 2009 in LA, and I was hooked. Later I got the Westone ES5 (wonderful) and then ES60 (phenomenal), as well as the JHA 16 Pro Freqphase (great right out of a phone) and their Roxanne (fantastic but needs to be amped for best results). I was so happy with those that I have stopped looking for better after that, and only got the RE-1000 later on as a beta tester.