Try www.bouncycastle.org. There are also some "hidden" classes shipped
with Sun's Java, but they are not documented and may therefore change
with java versions.

JK

You could use bouncy castle's ASN.1 parser. It's pretty good.
If you know what you'r doing, you can parse ASN.1 DER like
maybe something like this:

   /** Returns an array of [b|s] */
   static byte[] convertToRfc2437(byte[] signatureValue)
   {
    // ----------------------------------------------------------------
    // | 0x30 len | 0x02 len(r) | ... r ... | 0x02 len(s) | ... s ... |
    // ----------------------------------------------------------------
    int start = 4;
    int len = signatureValue[start - 1];
   
    byte[] data = new byte[len];
    System.arraycopy(signatureValue, start, data, 0, len);
    byte[] r = fit(data, 20);

    start += len + 2;
    len = signatureValue[start - 1];
    data = new byte[len];
    System.arraycopy(signatureValue, start, data, 0, len);
    byte[] s = fit(data, 20);

    byte[] b = new byte[40];
    System.arraycopy(r, 0, b, 0, 20);
    System.arraycopy(s, 0, b, 20, 20);

    return b;
   }

   /**
    * Fit (stretch or shrink) b into an array without losing precision,
    * as required by I2OSP.
    */
   private static byte[] fit(byte[] b, int length)
   {
       int len = b.length;
       int offset = (b[0] == 0) ? 1 : 0;
       len -= offset;

    if (len > length){
      // DSA value too large to fit
          // into 'length' bytes -- this should probably
          // be handled . . .
      len = length;
    }
    byte[] bytes = new byte[length];
       System.arraycopy(b, offset, bytes, length - len, len);
       return bytes;
   }