In ear monitors can work great for reducing the volume level on stage and clearing up the sound in the room. But you need to make sure you're using good headphones or earbuds if you're going to use this type of approach.
Here are some quick suggestions for in ear monitors that might work for you, depending on your needs and budget...
The Shure SE215-series is a great all-around option for a mix of quality and value. (About $100 on Amazon)
Selecting the right option
Just do some research and make sure you get the right headphones or earbuds for the right person.
A bass player or drummer will likely need a different set of earbuds than a singer or keyboard player.
(Drummers and bass guitarists are listening for much lower frequencies and articulation with their instruments, so the earbuds need to be able to reproduce those. Some musicians will even use "tactile transducer" monitors like the ButtKicker to add a silent subwoofer effect.)
Quality custom molded ear monitors work great for this, but you can get some good "universal fit" earbuds that do the trick as well.
Technology and Price
There are a lot of price and quality differences with in ear monitors. Some monitors use one driver, others use two or three or more.
More drivers are not necessarily better. It depends on what sound profile you want.
What you're really looking for is a tight and comfortable fit with a frequency response that works for your instrument/voice.
Use the gear that works best for you and your team, and fits in the budget that you can work with. Just make sure you set reasonable price and performance expectations.
If you want musician-quality ear monitors, you'll likely be spending $200-800 (or more) depending on the technical requirements, preferences, and options.
The big takeaway here is for you to try out some different options before you spend a lot of money earbuds so that you are making an informed decision. You won't want to use them if they don't sound right!
Here are a few more resources you may be interested in checking out:
Here's a nice detailed case study from Wave on some things to consider for worship in ear monitor systems.
This is a good primer from Collaborate Worship for how to use in ear monitors on a budget.
And here are some pro tips from Church Production Magazine for selecting the right ear monitor, mixing the monitor feed, and maintaining them.
I hope that helps clear up some mystery about in ear monitors and provides you with some helpful resources to get the right technology for your needs.