Do you mean the 3 character codes used in the GDSs, such as 319, 744, 738, EM7 ...? If so, I think you will find that there is a subset of generic codes published by IATA but each airline will supply its own variants to the GDSs when they supply information, which is why you will sometimes see two or more flight numbers and a/c types for the same physical flight in the case of codeshares and leases.
If you PM me I may be able to supply something that will help you, for $148! (That was a joke by the way.)