Fitting a new security door on existing frame

I hope that someone with some basic handyman knowledge can help me figure this out:
I currently have both a glass door and a screen door to my yard.  I would like to replace the screen door with a security screen door (ie, metal, with a lock). 1.       Can I assume that I can hang the new door on the existing frame, even though it’s a frame that fits two doors? 2.       Aside from Home Depot, are there other suggestions on where I can get a security screen door that isn’t too expensive?   Thanks, Elizabeth