Vans marshutni depart from Yerevan to Sevan a few times a day. You can get relatively close to the town by taking transportation heading for Vardenis, Dilijan, Ijevan or Noyemberian and getting off part way, but getting to town might be a little complicated with the latter three. Taxis from Yerevan or anywhere else are another option, at 100 drams .33 US$/km.