Roland's MPU-401 interfaces always seem to come in pairs. Consider the following:
The original MPU-401 Interface was an external unit that required an interface card for the particular computer it was supposed to connect to.
The MPU-IPC, MPU-IPC-T & MPU-IMC contained their Interfaces on ISA/MCA cards, but required and came with external breakout boxes for connecting midi equipment.
The LAPC-I operated in the exact same way, but the external breakout box was a separate purchase. Also, the LAPC-I already had a synthesis engine.
The SCC-1 and MPU-401AT also came with a synthesis engine or a connector to piggyback one, but required special cables to connect midi in and midi out.
In all cases except the first, the card is the vital part of the equasion. While the card itself is not very useful without the breakout box, the breakout box can be built using a few common components. Also, the breakout box is absolutely useless without the corresponding card (although I don't know any reason why you couldn't use the MPU-IPC-T's breakout box with the MPU-IPC or vice versa.) However, assuming you are only needing the essentials, then all you need are 3xDIN connectors (female), 2xceramic capacitators, 5xresistors, 1xdiode, 1xoptoisolator and 1x25-pin parallel cable or 9-pin serial cable.
In the first case, things are almost as easy. The ROM, RAM, interface and microcontroller of the MPU-401 are all housed in the external box, all you need is a simple interface card to use it. Now, for the PC, you will need an 8-bit ISA breadboard, 4x74 Logic Chips, 4xceramic capacitators, 1xresistor, and 1 DB-25 pin connector (female). On the plus side, the MPU-401 can be used in multiple computers, assuming you aren't using more than one at a time.
As for the last two, if you can't find the proper cable, you can build one out of a mini-DIN cable and a DIN cable, assuming you can figure out the pinout.