There are some resources that might be of interest to you (no affiliation)
Ideas: http://www.oppsdaily.com/
Already done successfully
Ideas: http://www.oppsdaily.com/
Already done successfully
I'd also add https://flippa.com to this list.
It's interesting to see which businesses are being put up for sale.
FYI it's https://nugget.one/daily - for the free daily startup idea :)