Mega:

Mini Meduino Mega 2560 R3 Board Pro Mini ATMEGA16U2 with Male header for Arduino

https://www.ebay.com/sch/i.html?_nkw=Mega2560+Pro+Mini+ATMEGA16U2+-ch340g

0.8" 0.8in 16 Segments display 27.7x20mm 0.8 inch
https://www.aliexpress.com/wholesale?SearchText=0.8"+0.8in+16+Segments+display+27.7x20mm+0.8+inch

Plugboard:

10 3.5mm Female Stereo Audio Jack Connector panel mount
https://www.ebay.com/sch/i.html?_nkw=10PCS+3.5mm+Female+Stereo+Audio+Jack+Connector+panel+mount

10 6" 3.5mm Synth Patch CABLE black
https://www.ebay.com/sch/i.html?_nkw=10+6"+3.5mm+Synth+Patch+CABLE+black

P6KE6.8 TVS DIODE
https://www.ebay.com/sch/i.html?_nkw=+P6KE6.8+TVS+DIODE

-connect ground (left pin) on plugboard to ground (left pin)  on main board with a 1K resistor in case a patch cable touches the jack and the ground pin of a TVS, the pin is not shorted to ground. 

Z30:

arduino nano v3 

https://www.ebay.com/sch/i.html?_nkw=arduino+nano+v3

0.56" 7 Segment LED Display Common cathode 

https://www.aliexpress.com/wholesale?SearchText=0.56"+7+Segment+LED+Display+Common+Cathode

Common Components:

5mm straw hat led
https://www.ebay.com/sch/i.html?_nkw=5mm+straw+hat+led

6x6x5 pushbutton blue / black / 1 red
https://www.ebay.com/sch/i.html?_nkw=6x6x5+pushbutton+blue

2 Position SPDT 1P2T 3 Pin PCB Panel Vertical Slide Switch
https://www.ebay.com/sch/i.html?_nkw=2+Position+SPDT+1P2T+3+Pin+PCB+Panel+Vertical+Slide+Switch

m2 standoff
https://www.ebay.com/sch/i.html?_nkw=m2+standoff

10pcs 3 Pin PCB Mount 5.5x2.1mm Female DC Power Jack Socket Connector V6Z5
https://www.ebay.com/sch/i.html?_nkw=10pcs+3+Pin+PCB+Mount+5.5x2.1mm+Female+DC+Power+Jack+Socket+Connector+V6Z5

copper 5.5mm x 2.1mm DC Power Jack Socket Female Panel Mount Connector plug
https://www.ebay.com/sch/i.html?_nkw=copper+5.5mm+x+2.1mm+DC+Power+Jack+Socket+Female+Panel+Mount+Connector+plug

4 AAA Cells Battery Holder
https://www.ebay.com/sch/i.html?_nkw=4+AAA+Cells+Battery+Holder