They are not hard to make yourself. I knew one guy who used to make quick custom made holsters at conventions and made a tidy sum. A long time ago I made a holster for a ST III phaser I had using his technique and it cost about $1.50 in materials since it was made in vinyl.
Also guess it depends on which phaser you are talking about as well. As I recall the STII phaser was put backwards into a pocket on the field coat.
Holsters for TNG and later-series props (phasers and tricorders) are usually made from vacuformed Kydex with a slight texture (similar to the plastic that TOS Communciator shells were made from), a front shell with a flat back panel. The flat back panel is attached to the costume with Alice clips.
In Trek 5, the Assault Phasers were not holstered, they were attached to the belts with black plastic "handcuff" strips.
In Trek 6 the props were just carried in hand, so again no holsters.
Since we're on the topic, back in the TOS days they used Velcro... strips of it on the side of the P1 (and in the case of "stunt" props, on the handle as well), attached to big panels of Velcro attached to the right hip or thigh.
Prior to that they experimented with black leather waist belts, and before that asymmetrically cut brown suede belts.