I’ve added a seatcounter.com link to the recommended link section.

Seat Counter is a great tool to generally see how many seats are left in a particular fare bucket on a flight or routing. The tool will show the available number of seats in each fare class. If a 9 is showing in the fare class that means there are 9 or more seats available. They also color classify availability in green, yellow, and red (green meaning high availability and red meaning low).

You can see from the search results that the only direct flight of the day from TPA to LAX is on Delta and has plenty of availability left including 5 tickets in discounted first fare bucket A.

For information on how to read the Seat Counter results see their FAQ here.