I make buttermilk at home either from the curd which too is homemade or with the cream that is collected over a period of 8 days (for the collected cream the starter or the initiator is a spoonful of curd to which is added the cream skimmed from over the boiled milk and this is kept in the fridge for as much time as you like, for me it takes about 8 days to get a substantial amount of buttermilk and butter, this is then churned after leaving it outside the fridge for about 5 hours or after adding about1/2 cup of warm water ).
Normally i make buttermilk kadhi or these dosas with the buttermilk that i get. These dosas are very versatile and you can use whatever you have with you the buttermilk helps in fermenting the batter and it does not have to be kept standing for a long time so you get instant dosas in no time.
In the above dosas i have used rice flour and semolina , the proportion varies as it does not matter what or how much of each you use . Take the flours in a vessel and add enough buttermilk to give the batter a dosa consistency .
Make dosas on a NON-STICK PAN just the way you do the other dosas