Hi and welcome,
it partly depends on where you´re located and what electronics dealers you have access to.
PCBs you can get from SmashTV. Here you can either get a "standard" (Wilba) SEQV4 PCB, although I think it is not available at the moment.
Or you can get seperate DIN, DOUT, DIO-MATRIX PCBs, if you plan to build a custom SEQV4.
Either way, you definitely need a LPC17 CORE PCB from Smash and a LPCXpresso board from Mouser/Watterott/Lextronic/Ebay... depending on where you´re located.
There will be another, faster CORE MODUL available in the future, but I´m not sure, when this will be. For now, the LPC17 CORE will be the one you need.
If you choose to build a "standard" SEQV4, then you have to get some specific parts which fit this specific hardware design (like buttons, caps, encoders). More info about those specific parts and where to buy them can be found on Wilbas wiki page here. There you can also find more infos about the hardware design and how to build it.
If you choose to build a custom SEQV4 (after your own design ideas), then you´re more or less free to buy any kind of buttons, encoders, ... that suits your functional and aesthetic needs (within the regular Midibox hardware requirements of course).
For the rest of the electronic components (resistors, diodes,....), you can refer to any kind of electronics dealer of your choosing, depending on where you´re located. If you want to build a custom SEQV4 you can normally buy the other parts (buttons, encoders,...) there too.
You also need two 2x40 character lcds (HD44780 compatible) which can be best found on ebay for a reasonable price.
Hope it helps. If you have more questions, just ask.
my regards