Adultpacifier.com. They have genuine quality adult-sized nipples (Size 7) and guards. Their stuff is quality, available in both latex and silicone (latex is better if you're not allergic) and doesn't have sharp edges like other brands do. Cheaper brands only have up to size 6 nipples and use baby-sized guards that don't fit an adult well.
I use the BabyPants ones at https://www.comfycare.net/pacifiers. They are relatively cheap, 3/$17.50. Their basic and no frills but they have a NUK 6 nipple and a decent size shield. My favorite ABDL Store is Big Tots on Etsy. You may also find pacifiers on Etsy from small, independent vendors.
I don't like the BabyPants one. The nipple is hard silicone and has as sharp molded edge on it, and is still a bit too small for an adult mouth to suck completely comfortably. Their pacis also don't "breathe" without modification. For me, the extra investment was WELL worth it.