The shortest translation would be apropos
in the context of reminding somebody of something while talking about a similar subject.
Wir müssen noch einen Weihnachtsbaum kaufen!
Apropos, wir brauchen auch noch Christbaumschmuck.
If you really really want to say that you actually thought about something more closely and came to a conclusion, you'd say:
Ich habe darüber nachgedacht, und ...
Wo ich so darüber nachdenke ...
If something jumps to your mind and you want to tell someone because it is somewhat important, you'd actually say:
Jetzt, wo ich gerade dran denke ...
Mir fällt gerade ein: ...