I am no expert, in fact have been trying to master the code now for most of a year (off and on, not steady). I am currently on a plan to have the alphabet, prosigns and punctuation mastered by the end of January, so maybe we can have some QSO's later.
Everything I have read so far says learn to copy well before trying to send. The theory is that once you can "hear" code well, then your sending will be better. This is even more true in Straight Key sending where the rhythm of the dit and dah spacing is completely up to you.
Good for you to have mastered a large part of the alphabet at 15 wpm. For straight key QSO's that is a fine speed to operate at (again from what I read and have listened to).
Another source of code to listen to once you have the rest of the alphabet is the ARRL Code Practice. You can listen over the air, the schedule is here: http://www.arrl.org/w1aw-operating-schedule
They also have mp3 files you can listen to over the web. http://www.arrl.org/code-practice-files
My vote is to get the rest of the alphabet and a few punctuation and prosigns mastered at 15wpm before anything else. Then your choice either go back and try to get to 20wpm and/or try some on-air QSO's.
Good luck and keep at it.
John - KM4FOS