Look through the more general "Bongs & Waterpipes" section as well. 14mm is just becoming more popular on smaller bubblers. There are a lot of smaller items in the bongs section that are not complicated recyclers and don't have complicated percolators, which is good for vaping. And you'll probably find more 18mm there as well. More of what you're going to find in the bubbler section is more geared toward being a "dab rig".
But yeah, 14mm is a lot more common these days, especially in the smaller size pieces that we use for vapor. Less people are looking for 18mm, and there are less available.
This is an interesting 18mm low-cost little bubbler that's on sale for $35 right now:
https://www.grasscity.com/us_en/ws-...fuser-bubbler.html?nosto=nosto-page-category2