Yes, we support PostGIS. We can install it on shared, reseller, and managed dedicated servers by request. Open a support ticket with our Guru Crew at http://my.a2hosting.com and we will enable it for you. Please include in your ticket the names of the PostgreSQL databases that you want to use with PostGIS.
Of course, you can install PostGIS yourself on a VPS account.
For more information about PostGIS, please visit http://postgis.net.