For now, I would not worry too much about your source. IEMs are not super power hungry, so power shouldn't be an issue in most cases. The main reason dedicated digital audio players are better is that they tend to clean up electronic circuit noise generated by cellphones and offer compatibility features for a wider range of audio gear. That, and parametric equalizers can be easier to use depending on the player.
The headphones/IEMs are the most important part of the audio chain starting out, so I suggest just focusing on getting that part down. If you have an issue like crackling, not enough power, battery troubles with your phone, compressed dynamic range, etc, then you should consider a dedicated audio source.
One thing I do suggest is getting a good audio player app to make sure you are utilizing your hardware without a software bottleneck. I use Neutron music player, other players are also capable of this.
If you are looking for headphones, chances are you will need to start investing in better sources because of the power requirements. Planar magnetics generally need a lot of power, and some headphones have a lot of impedance, so a digital audio player with a strong amp makes sense.
Headphones are a bit more forgiving, but this is an interesting case to consider.
@InvisibleInk suggested Hifiman as an option for you, and I agree that they are pretty good, but I found the HE400se I have to be unnatural to my ears because of the tuning. I ended up settling for the Beyerdynamic DT 1990 Pro as my reference standard because the pinna gain region and lightly elevated treble response presents music the way I hear it live tonally speaking, whereas the He400se was too dark and subdued to me. Many people consider that same set natural sounding, and it took me some digging to figure out this discrepancy.