2 suggestions
Play it again sports sells used catchers gear. Its where I bought the stuff I use when I catch for my daughter
Or, ask around at a 12u team if their catcher has gear she has outgrown. You might find some gear for sale, or they might just give it to you. When my dd was younger she primarily caught and I gave her first set of gear to the 10u team in our organization so they could let girls who were just starting out have something to use.