We got 12 guineas last year through Blue Seal. They often have keets but they go fast, so we got lucky as we asked late. The Blue Seal in Chichester seems to have less of a backlog or orders (ie, Concord Agway sold out before the keets even arrived), so we actually found some there for the taking. There are lots of web sources, too. We raised ours with our chicks and they all roost together in the same coop now. They need to start on game bird crumbles, as the chick starter is too much protein for them, but once they are about four-six months old they can all go onto egg layer.
Ours are a year old now and didn't do anything for the ticks the first year, as they were penned in the brooder all summer. They're hilarious and noisy birds, but we hope for good results with the ticks this year. We have 20 acres and only nine guineas (three disappeared, and we suspect a fox or coyote, but all we found were feathers in a far field), but they spend a lot of time all over the property so fingers crossed they're eating enough ticks to make a difference. They are roamers, though - if you have neighbors, warn them that they will have guineas. We can't keep ours out of the neighbor's yards. They're good fliers, and they like to roam. We try to tell the neighbors that the ticks being eaten is worth the noise the guineas make, but some aren't pleased all the same. :/
Good luck!