The two express bus operators on Malaga to Almería are Alsa (www.alsa.es) and Avanza (www.avanza.es).
For information in English click "en" or the Union Jack. Both operate between Malaga (Maria Zambrano) bus station and Almería.
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Alsa buses are blue and Avanza are red and white. Typical express journey time just under 3 hours.
Other companies provide "stopping" buses which take much longer but which give you a chance to stretch your legs or have a smoke or a coffee.
Even though Alicante is slightly nearer Roquetas, Malaga is better served by flights from UK and Ireland - and the bus connections are more frequent.
If it appeals to you, and you're not in a hurry, from Malaga you also have the alternative of stopping over in Granada if you want to travel by rail. Malaga bus and train stations are right next door to each other. If you're very lucky you'll be using the amazing scenic old railway line from Malaga through Bobbadilla to Granada and the scenic route from Granada through Guadix to Almería. Allow 5-6 hours travel time, but keep your camera very close to hand!
(By the way, the section of railway between Malaga and Bobbadilla was built by British engineers in the mid 19th century, and it is a testament to the quality of the work that the line is in everyday use today.)