Most light fixtures expect a 4" "ceiling box" which is a kind of rounded-off square box (also called an "octagon box"):
I always mount them as you described -- on a 2x4 in the joist cavity. A similar box is available for about 3x the price called a "ceiling/chandelier box" and comes with hardware for mounting fixtures like fans or (!) chandeliers, but the weight of your fixtures will determine which you need.
Get the kind of box that has knockouts in the bottom (shown), since you won't be able to bring the cables in to the side of the box. Drill or saw a hole in the 2x4 to match up with the bushing in the box and pass the cable through the hole.
They make these boxes with a mounting tab attached, which would eliminate the requirement for the 2x4 (just mount it to the side of a joist), but I don't like them -- you're limited in where you can mount the box, and I wouldn't trust them with any weight:
Do you have access to the space above the ceiling?