You can get 18650 battery holders on eBay; there is even a box for 2 18650s on eBay that people on ECF have used for modding.
....whew, found it on eBay

-
1pc 18650 Battery Case 2 x Parallel Connection DIY Case | eBay
The main difference between using 14500 or 18650 for a mod will be battery life. I built a vv mod using a LM2596 step-down module I got on eBay and using 2 14500 Trustfire
batteries. These batteries last me most if not all day - I vape ~3mL at ~4.5V and 3.2ohm carto.
For a single battery - 3.7V mod - an 18650 should last most vapers most, if not all, of a day. While with a single battery mod with a 14500 battery, you may have to switch batteries during the day.
So
vaping time and size will be the deciding factors for which type of battery to use.
Or, if you like, just build a mod for each type of battery.
