Since that model has been discontinued finding parts for it will be difficult. You could try bearcom.com They are a commercial radio vendor out of Texas that I have done some business with in the past and recently. They might be able to help you find a longer antenna. I looked around online but am only finding the stock antenna and the stubby one which will just make things worse. The problem with any pack set is your body is blocking about 50% of the signal. A longer antenna would help a little bit but your body will still be in the way of the signal. If you turn your body so it is not between you and the other person it is possible they will hear you better.
If you are near a ham radio shop you might be able to get one of those places to find an antenna that will fit your radio. The problem with those though is they are designed for ham radio channels and not commercial channels. There is not much of a difference between the two but it is enough difference that it can effect your rage and your radio might heat up more then usual too. This also depends on the quality of the antenna too. There is a lot of cheap Chinese junk out there these days. Diamond or Comet is usually a good brand.

Hope that helps some. Hopefully some of the ham radio guys that are on here will chime in. You will get a lot of different opinions on the subject which is all good information to have.