RobynOrdDo you mean cartridges? You can find them on jetpens.com or Amazon. Just search for Pilot ink cartridges, or any International Standard size fountain pen cartridge.
If you're talking about ink in general, to use with the included squeeze converter, then you should be able to use whatever ink you like.
ardichokeActually, the Metropolitan (as sold in the US) does NOT accept standard international cartridges. Thus my earlier request that Massdrop's description wording be a bit more specific. The Metropolitan DOES accept the usual Pilot cartridge. In fact, it comes with one in addition to the squeeze converter.
But with the squeeze converter, one can use any fountain pen ink. Alternatively, one can either buy more Pilot cartridges or refill the one included. Once used up, the cartridge is relatively easily refilled with a syringe and blunt tipped needle. Using a needle without a sharp edge is important. Well, for me it is, but I have shaky hands sometimes.
Hope this helps.
Cheers.