The good news: there's lots of options to consider.
HP followed up the 48 series with the 49G, 49g+, 48gII, and 50g. All of those are also out of production, but like the 48gx, used ones are fairly easy to find in the usual online auction sites and sellers of used equipment.
The Prime would be the only currently-produced HP calculator with a similar feature set (actually a much greater feature set). The operating model is quite different from the 48gx, however. You should take a close look at how the Prime operates before making a purchasing decision in that direction. The Prime is not code-compatible with any other series, so you'd have to recode any programs you previously used on the 48gx.
The 50g would be a great choice if you want something that operates in a similar fashion to your 48gx, but much faster and with enhanced features (more memory, SD card, larger display). UserRPL code written for the 48gx usually works with little or no alteration on a 50g. The keyboard layout is different, though, and would require some time for transition. If you don't need the additional resources, a used 48g or gx would provide instant familiarity and "feel" for you.
As was said above, the 48 is not made anymore, and neither do any of its direct successors (49, 50)... If you want to continue with the 48 experience/use model, I would suggest getting a 2nd hand 50G...
The current top model HP calculator is HP Prime. A successor of the HP38, itself a derivate of the 48...
Honneslty, I am an old 48 user but now use exclusively my Prime (but I migth have a "slight" biais:-)) .
Prime is way more powerful than the 48, faster, build in computer algebra system, better screen, OK keyboard, is nice and slim, good programming language, lots of memory... and a triangle solver!
What you will loose is RPN and RPL. And for some, this is a deal breaker (althrough there is a RPN mode, it is relatively limited).
The other thing that you loose is the simplicity/speed by which the 48 can handle units and units conversion.
But all in all, Prime is, in my opinion better. You can give it a "try" by downloading the free android, IOS or windows Store apps.