Imagine i'm walking in the street starting looking for a florist and i come across one straightaway. Should I say or think: "How lucky!" or rather "How lucky for me! or How lucky I am? Is there any other phrase that would be more appropriate and colloquial to say in this situation?
"How lucky!" sounds a little bit old-fashioned or overly-dramatic.
"That was lucky!" or "that's lucky!" would probably be the most natural-sounding.
(Upon immediately finding something "lucky" it doesn't really matter too much whether you use the past or present tense, as you could either be referring to the event that has just happened or commenting on your present situation)