Where do I start? Want to put a bathtub in

Hi all, I'm not sure where to start. I own a property that isnt that old, a few years. We built it. Well I am a bit annoyed at myself because I have an en-suite with a double-shower but I should have put a spa bath (rectangular not corner) in with a shower-head over the top.

Where is the best place to get some prices to convert the double-shower into a bathtub? I dont need to renovate the whole bathroom just want to put a tub in. Any idea what kind of $$ I'd be looking at also? I'd need someone to do the work for me. Not a handyman myself.