Just the needle caps are very hard to find. I searched and searched about a year ago. I found some that were pretty expensive, and since I didn't know the specific dimension and neck threading of my bottle necks...I didn't buy any of them. Would have been a waste of $$ trying to find the correct ones to fit, IMO.
What I ended up buying, was some of these little 5mL bottles with the needle caps from FT:
$2.31 E-liquid Refiller Bottle for Electronic Cigarette (10-Pack / 5mL) 10-pack at FastTech - Worldwide Free Shipping
I just took the caps off to use on my other larger bottles that are 30mL and 50mL. They fit mine fine, but IDK if they'll fit
all of those size plastic bottles. I bought the little 5mL ones, instead of more expensive sizes...since the caps appear to be the same no matter the capacity of the bottle.
You can measure the mm diameter of any bottle opening (that's the easy part), but...the neck threading is the part that I don't know how to determine.