Both. Some people re-sell other people's drivers into custom moulds. E.g. http://www.cosmicears.com/product/r0101/
Some people make their own drivers/housing e.g. https://alclair.com/ http://www.jhaudio.com/ https://www.acscustom.com/uk/products/in-ear-monitors
Some people can do custom eartips for pre-existing IEM's. e.g. https://snugsearphones.co.uk/
Alclair's are meant to be very good for the money.
Interesting.... Definitely some investigation needed....
@JB started
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Both. Some people re-sell other people's drivers into custom moulds. E.g. http://www.cosmicears.com/product/r0101/
Some people make their own drivers/housing e.g. https://alclair.com/
http://www.jhaudio.com/
https://www.acscustom.com/uk/products/in-ear-monitors
Some people can do custom eartips for pre-existing IEM's. e.g. https://snugsearphones.co.uk/
Alclair's are meant to be very good for the money.