function GetPhNameByAlias(sAlias: string): string;
var
Database: TDatabase;
pszDir: PChar;
begin
Database := TDatabase.Create(nil);

pszDir := StrAlloc(255);
try
Database.AliasName := sAlias;
Database.DatabaseName := 'TEMP';
Database.Connected := True;
DbiGetDirectory(Database.Handle, True, pszDir);
Database.Connected := False;
Result := StrPas(pszDir);
finally
Database.Free;
StrDispose(pszDir);
end;
end;

PS. В uses нужно прописать BDE.