It's actually pretty realistic. The percentage of Ur-Priests and Sublime Chords in the world is going to be DRASTICALLY less than that of Sorcerers and Clerics. The majority of scrolls and wands would be crafted by the more common classes. If you wanted to buy a scroll from an Ur-Priest, I would ask you to explain how you found one, and convinced them to sell you a scroll. They aren't exactly a dime-a-dozen, especially in a world where not worshiping a god consigns you to an eternity of torment in the Abyss.
I mean, if the campaign was centered around them? Or your character WAS one? Absolutely. But in most settings, you're not going to find an Ur-Priest in your friendly neighborhood magic shop. You're going to find a Wizard.