You can get one for a reasonable price at eBay. The one on the right--the 50g--is the one to get these days. The middle one is basically a failed update of the one to the left, and the left one--the 48G--is rather old now. I bought mine in 1995, I think.
BTW, you are better off with a TI graphing calculator unless you like the
RPN entry mode. HP calculators are known for their RPN implementations, which is great if you need to be able to perform exploratory calculation, but it does take some getting used to.