I'm pretty good at finding things on line..but this stumped me. All I could find was China suppliers for this type. You may have to buy extra carts from where ever you got them in the first place.
But why buy so many extra carts when you can refill what you have? You can but
e-liquid from tons of places and filler for the carts you make from teabags, fluval fish
tank filler, polyfill and about a dozen different things. There are countless posts about this very thing.
Good luck and welcome to EFC!