It might actually be cheaper for you to buy a set of
Paul Thumbies than the appropriate shifters, depending on your exact set up. You would re-use the bar end shift lever itself, but not the mount that places them in the bar end.
Or you could always use a pair of cheap thumb shifters,
Sunrace makes one that retails for about $10, it's a great choice if you don't mind friction shifting, but they aren't as cool as the Paul Thumbies.
Wait...18 speeds, meaning a triple with a 6 speed cassette, or a double with a 9 speed cassette, because that makes a big difference. If you have a 6 speed cassette, indexed components will be hard to find, but I doubt that a new bike would come with them.