The main sites I use are Mouser and Tayda. Tayda is generally cheaper and is good for the more common components(resistors, caps, jacks, etc).
Mouser is more expensive and the search function takes ages to navigate but they have pretty much everything you can imagine. A quick look at Tayda and they seem to have everything except the transistors, pots, and mosfets. Mouser should have the transistors and mosfets. Pots are a little more difficult - Tayda has them but are almost always out of stock. There is a thread on here about pots but I can’t find it at the moment. I’ve not built this module myself so don’t know if you can substitute from the ‘recommended components’ in the BOM.
https://eu.mouser.com
https://taydaelectronics.com
Hope that helps 
Edit: thread on potato pots is here: