Dont ovethink it or worry...they break off their mount and fall to the bottom. Unles it disentegrated into tiny pieces [highly unlikely]...its down inside.
A simple google search with a few BMW specific key words should bring up images. I know Bluebee and othes have cut e39 & e38 coolant tanks in half...the e53 tank has same design.
EDIT: I just typed "BMW Coolant tank float broke off" into a google search box and clicked images...got hundreds of pics and links to past same issue
No, they arent sold separately...do the google search for visual.
