Where can l get a body kit conversion for the 03' element is plastic fenders and bumpers to the steel 07'-style body?

Or...

Could l just take out the plastic molds and just get some 06-07' version fenders from the junk yard and just swap them?

Good luck finding 07 Elements in a scrap yard with 4 good fenders.
The front ones will be different than the ones that are on the 2003 model.
The rear 1/4 panels will fit without any issues.

You likely will need to purchase the fronts and rears from Honda.

they are not steel, they are the same plastic parts just painted.