What's a good P to use in ElGamal key generation?
With most finite field crypto using a standard group is no problem. I think ElGamal encryption doesn't have any specific requirements for group or generator that plain Diffie-Hellman doesn't have.

So you could look into RFC 2409  Internet Key Exchange or RFC 5114  Additional Diffie-Hellman Groups for Use with IETF Standards for groups.

AFAIK ElGamal signatures have some additional requirements for the generator, so the above groups are probably a bad idea. But I wouldn't use ElGamal signatures in the first place, preferring DSA or Schnorr.