Further to what Marze said, inks form your local art supplies store (Eckersley's???). Just be sure they're specifically water-based or alcohol-based inks. The alcohol ones will dilute in metho.
Another option would a wood turner's supply store (or similar). They should have a range of timber dyes. The one I've used is U-Beaut water-based dyes. Not all suppliers sell individual colours, so you may have to buy a whole pack. I paid $39 for a 7-pack, but having an assortment of colours was fine for me.
FWIW, alcohol-based inks/dyes will raise the grain less than water-based, but either will work. Just personal preference.