The following bus companies operate Lisbon to Seville:
Daily Alsa.es
1) www.alsa.es (At least twice daily )
2) www.damas-sa.es (three times a week.)
See Also this site.
http://www.seviquorum.com/transportes/autobuses/provinciales/index.htm
Alsa.es appear to offer the best frequency and service.
In Lisbon you get the bus from the Gare de Oriente Bus Station twice a day at:
Dept: Lisbon 11:30 Arrive Seville 19:45
Dept :Lisbon 20:45 Arrive Seville 05:45 next morning)
Price quoted is € 40.00
Please note that this information is based on currently available information on the web and may change at any time. I recommend that you check at Gare de Oriente Bus Station well before travelling for changes.