This is based in the comments of some redditors. I like to have my players to seek in the book the item they want but you have to keep something in mind: IT IS IMPOSSIBLE for a merchant to have EVERYTHING listed in the books so there’s a 50% chance they have it. This reflects in a DC 50 with a 1d100 check. Some modifiers +10 for each level of the wanted item above the character’s. Or -10 for each level below the character’s level.
Here’s a page with this explained and with some examples. Have fun!