Parts
If you are just purchasing generic parts All Electronics has an excellent selection. To squeeze out better sound you will find better quality parts at
http://www.digikey.com
DigiKey is an excellent source for reasonably priced Panasonic electolytic capacitors. They also have shipping discounts on moderate size orders ($25 US). There catalog is much greater than that of All Electronics.
Another source for parts is
http://www.partsconnexion.com
Boutique parts such as BlackGate capacitors, etc are avaialable here.
There are suppliers closer to you in Japan, Austaralia, Singapore, and Hong Kong, all of which sell better quality parts than All Electronics. I have purchased a lot of parts from All Electronics and I have been satisfied with those purchases, but I prefer to use better quality parts in the signal path. There is no way of knowing what you will get from All Electronics until it arrives.
Considering the relative inexpensive cost of the Sonic Impact amp and the excellent sound that comes from such a prouct; the cost for tht few parts that one needs for boutique parts is minimal. The sound enhancement from top shelf parts in, IMHO, is worth the extra pennies for these sonic dynamoes.