Goodness what a question. You can see lemurs all over Madagascar. The first place you can see them upon arrival in the country is at Parc Tsimbazaza, which is the national zoo and botanic gardens, right in the capital city. Next you can go on a day trip to see the Indri indri, the biggest lemur, at the Andasibe National Park which is a half day drive from Antananarivo. You can stay overnight there. Then there are national parks and private reserves all over Madagascar where you can see lemurs either in the wild, or, in the case of private reserves, usually free to roam but treated like pets. Some of the reserves that you can visit are Nahampoana Reserve and Sainte Luce Reserve (www.sainte-luce-reserve.org) both of which can be reached easily from Fort Dauphin in the south east. For Madagascar National Parks, you will need to book through travel agents or if you prefer to travel alone and plan as you go, you can by tickets at the entrance of just about all Madagascar National Parks. about 8 years ago
You can see them everywhere really (in the Nacional Parcs).
Specifcaly, you can see the Indri (biggest Lemur) in parc nacional dAndasibe-Mantadia which is only a few hours drive from Antannarivo
You can also see them up close in Reserva Speciale de lAnkarana - where you see the tsingis - the Lemurs (sadly) are almost tame there and come up to you (try not to feed them) about 8 years ago