Instead of shelling out a ton of money ($1,000+) for custom molded in ear monitor headphones, check out the subReddit DIY In Ear Monitors:
https://www.reddit.com/r/DIEMs/By the way, you can even 3D print your in ear monitors, but this is absolutely not necessary, and especially so if you are cheap.
Getting ear impressions from an audiologist generally costs $50-100 but I prefer to do it myself.
It is also not difficult to make proper ear impressions, too. See this for example: http://static.speedwaymotors.com/pdf/910150509.pdf
However, this was my basic search technique to find this document about making your own ear impressions: "impressions" AND "ear" AND "bite block" AND "make" filetype:pdf (I hope this allows you to do more research)
There is a great guide on making basically perfect DIY in ear monitors, pay attention to the first comment (however it involves 3D printing...also I much prefer clear shells for the in ear monitors...) on this Reddit page: https://www.reddit.com/r/DIEMs/comments/kfdwf5/made_my_first...
Anyways, one should really take a look.
When it comes to audio quality and listening on a Bluetooth phone without a 3.5 mm jack, you use a DAC (digital to analog converter) which allows you to enjoy the music on a totally new level. I would recommend this one: https://www.amazon.com/FiiO-Balanced-Amplifier-Windows-Porta...
There are also Bluetooth DACs that may be preferable too. I recommend the FiiO brand. Here is an example of a Bluetooth DAC: https://www.amazon.com/FiiO-BTR3K-Bluetooth-Headphone-Unbala...
If you enjoy music a ton just in general and love wired headphones, I strongly recommend getting a DAC that is compatible with mobile phones even if you are not interested in DIY in ear monitors...
When it comes to making phone calls (or even microphone use) with DIY in ear monitors, you need to be observant of the type of 3.5 mm plug that you use. ONLY plugs with 3 indentation lines (NOT 2!!!) are capable of microphone use. Anyways, here is an example of what I am talking about so we are clear on this issue: https://duckduckgo.com/?q=3.5.+mm+plug+microphone&t=fpas&iax...
When it comes to using your in ear monitors for phone calls or microphone use you need to:
1. Get a Bluetooth transceiver that is mic capable, such as this: https://www.amazon.com/Bluetooth-RollingStone-Headphones-Str...
2. Get an in-line microphone which plugs in directly to your DAC and gives it instant microphone capabilities
* Android version: https://www.amazon.com/Bose-QuietComfort-Headphones-Samsung-...
* iPhone version: https://www.amazon.com/Bose-Comfort-Headphones-Inline-device...
I hope this post helps.