Obtain 384 bits from a 256bit MasterSecret
You don't actually need 384 bits of key material.  The IV for GCM does not need to be secret, and may be chosen deterministically, e.g. as an incremental counter.  Thus, you only need 256 bits for the AES key, which you already have.

That said, if you did actually need more key material, you could use any standard KDF to expand your 256 bits.  Since you presumably already have a 256-bit hash function available (for PBKDF2), I'd personally suggest using HKDF-Expand.  Since your master secret is already a pseudorandom bit string, you may safely skip the Extract part of HKDF; see §3.3 of the RFC.

(You could also request more bytes directly from PBKDF2, but this is rather needlessly inefficient.  If you need more than one hash block of output from PBKDF2, it's better to combine it with a key-based KDF such as HKDF.  Technically, you could also use a second PBKDF2 pass with an iteration count of 1 instead of HKDF, but this feels a bit "untidy", as PBKDF2 was not really designed to be used like this.)