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I think what you want is RACE, Rapid Amplification of CDNA Ends. This lets you PCR from a known sequence to the ends of the RNA transcript. There are kits sold to amplify to the poly A, and/or the 5' capped end.
Did you already check Genbank and the EST and RNA transcript libraries? If you have a protein sequence, you can do a protein to nucleotide BLAST search, tblastn, I think it's called.