There's a place on their website that shows what cartos will work with the different size tanks that they offer. Basically, I believe there are only two lengths of cartos made (reg and XL). You can order a Phiniac that will fit a carto size that you like, or, buy cartos to fit the size Phiniac that you like.
Just be sure not to use a carto that is shorter than the height of the tank. There may be some overhang of some of the cartos, which is not so much of a bad thing, aesthetics aside.
Most e-cig distributors sell carto punch tools in their accessory sections. Like everything else, the quality of the tool can vary, as do their price. I have not personally used any so I can't recommend any. A few distributors also sell pre-punched cartos for a little more than their regular cartos.
Some users slot their cartos with a dremel tool device instead of punching, but most distributors discourage that practice as they believe the sharp edges can ruin the important 0-rings that keep the juice from leaking.