So far I know of only one replacement floppy for the MPC3000, the Teac FD-235HF and only certain models of these. The more expensive version of these Teac's have movable jumpers on them and range anywhere from $50 to $150 depending on where you purchase them. The cheaper versions, about $20 do not have these jumpers so I would not even bother with these. I occasionally run across the floppies at the local used computer shops or computer trade shows in the cheap floppy bins for just a couple of dollars each. Well worth the risk of finding a working one. They are pretty easy to spot since they have a jumper block next to the power supply connector.
_ or | signifies jumper across two pins
Note: FG jumper pins may be vertical or horizontal