This is a hard question to answer, because it would not be something I would ever say to someone. I would more likely say something noncommittal, for example:
B1: I just happened to see it in a shop window while I was strolling through town. I hope you like it.
If you really want to say something similar to what is written between the parentheses, I guess you could say:
B2: Never mind where I got it, just take it. (Gosh, this sounds so rude to me, though. It almost sounds like you stole it so you do not want the person to know where you got it from).
Maybe a slightly more polite version would be:
B3: Oh, it's not important where I got it, I just hope you like it and will make good use of it.
Still… I would probably say B1 over any other choice.