Wendy's advice is probably the best and the most logical - ask at takeaways - but if you want the authentic stuff then you'll need to get a book by someone like Madhur Jaffrey.
Go to www.curryhouse.co.uk It is the best recipes for t/away style curries by far. You can make up the base sauce and freeze it, I think I x by 4 and then it is always ready to go.
Emjt is right and I know coz I have made them and they are terrific, very authentic indeed. Every indian curry is made from a basic curry sauce and you add whatever to make different types e.g. onions for dopiaza, tomatoes for bhoona etc.