Use /filter to get all matching lines to another file (or @window):

Code:
/filter -ffc $+(",$todayslog,") temp.txt *<sour>*


Then $read(temp.txt,n) to get a random line from it

*<sour>* could also locate <sour> in the middle of other people's messages. If you're worried about this you could improve your wildmatch to '& <sour> *' (for example) where & may represent your timestamp