I came across a site called AAutobuses.com.
El portal aautobuses.com le ofrece información sobre las principales
líneas regulares de autobuses de nuestro país.
The site isn't very pretty, and it's only in Spanish*.
Anyway, AAutobuses seems reasonably comprehensive. Compared to the other options presented, it is better:
- If Movelia works for some route (e.g. Logroño - Bilbao), then the information provided is more complete (= includes price, etc) than that of AAutobuses. But for lots of routes Movelia is just useless.
- With Budbus it just depends on whether someone has happened to enter the timetables you're looking for. AAutobuses was better or equal for everything I tried.
(Some of my test searches: Santander–Bilbao, Sevilla–Cádiz, Valencia–Madrid, Vigo–La Coruña, Logroño–Bilbao.)
*That shouldn't matter much, as long as you understand what something like
LMXJVSD means on the timetables. If you don't, here's the key:
L = Monday (lunes)
M = Tuesday (martes)
X = Wednesday (miércoles)
J = Thursday (jueves)
V = Friday (viernes)
S = Saturday (sábado)
D = Sunday (domingo)