You should already have it in your MPC2500, I seem to remember you just open it up and slot the hard drive in. I believe it was just an upgrade for the MPC1000. It's now discontinued but MPC Stuff make a replacement version , you should ask them if it's suitable for an MPC2500
I'm pretty sure as the hole for the hard disk drive is the same size for both the 1000 and 2500, it should pretty much work on the 2500 as well. But like MPC Tutor said it worth speaking to mike at MPCStuff to double check the compatibility.
MPC 1000|128MB Ram, 100GB HDD, JJOS 2XL MPC 2500|128MB Ram, 40GB HDD, CD Drive, JJOS-XL MPC 2000| 32MB Ram, Version 2 LCD, 8-outputs & EB16 FX Expansion