Dungeon Fantasy 15 has a number of spellcaster templates, for both 62-point helpers and 125-point henchmen.
In my own game, I tend to make caster hirelings much less commonly available than non-caster hirelings.
I have a few reasons for this.
Difficulty of making them
GURPS character generation is a lot of fun, and I love what you can do with it, but making a list of 15-30 spells is not as fun for me. It's tough to make a good set of choices when you're making your own guy. NPCs can be more throwaway but they need to be effective in their niche. So they take time to make. While someone can hire an archer and I can pretty much go from template to loaded out NPC with a personality in a few minutes, if that, a caster will take me more time. I'm loath to spend that time if I don't need to, and I'm really unhappy spending that time at game (especially as I also field requests and questions about actual PCs.)
Usually people don't know they want or need a caster until the day of game. "We need a wizard of any kind" might be answerable with a 125-point Apprentice, if I can sit down and make one quickly. Or if I have one handy. What if there are specific requests? When you add on "with See Invisible and Levitate and enough FP to get us all over the castle wall" it's a bigger problem. Then I need a fully-made wizard with certain spells. "No" ends up being easier than "make a roll, and if you roll well, everyone leave me alone for 10 minutes while I make up a guy."
No matter what, once a caster gets introduced it's common for people to start seeing them as full-fledged master (whatevers.) Acolyte? Cleric. Apprentice necromancer? Master necromancer. Beginner elementalist with Stone Missile-13? 18d Stone Missile Sniper capable of firing into melee without hesitation. I exaggerate a bit, but not by much. The expectations rise. A 125-point guy who serves as a bandage for a serious gap in available spellcasting needs gets seen as a full-out replacement for a 250-point delver.
On top of that, skill expectations are high - all casters getting a -1 to energy cost for skill, able to maintain spells for free, and succeeding in casting despite penalties? That's a feature of well-made 250-point delvers. That's where a good measure of those extra 125 points go.
Annoying the PCs
If you do get a good, effective, well-designed caster, then it tends to eat into what the PC casters do. This simultaneously annoys the player due to overlap, and makes for less likelihood the NPC gets used. "We already have a necromancer" or "We already have an artillery mage" means the guy doesn't get hired. Or it means he gets hired and people choose to use their non-caster PC foe the session, turning this back into the High Expectations issue.
This tends to be why you don't see a lot of caster hirelings in my games, and see the same ones over and over once you do. I add in game explanations, too - casters being less common, wizards being more suspicious, the church being unhappy that delvers show up every three months saying, "Funny story, Father so-and-so died and was eaten, who else you got?" But a good portion of it is the mechanical issues of making them up, designing them well, having them ready to go, and then the expectations issue.