Congratulations on starting a computer business there are many ways to stock your store with parts and computers. I would first become a member of NASBA and then you can find some channels to use. I am a member of NASBA. Check out the flooring plans(they will stock your whole store for you with a monthly fee, as you sell the items they will send new ones to replace) but be careful as you can get into debt really quick going this route. You can also have a bunch of outdated computer equipment that you can never recoup your cost. But if you have a good volume of traffic this wont be a worry. I am not affiliated with any floor planners as my budget when I started my business was not big enough.
I am like most computer repair shops I have a few new cases some new MB's a few of this a few of that and a whole bunch of parts salvaged from older computers. When I need something that I dont have I pay the premium and go to newegg.com or geeks.com and get it express shipped. Customer satisfaction is the motivator for the express shipping as a happy customer sends more customers. A great place to salvage used parts is from the goodwill and the salvation army.
Overclockers do a wholesale section or you can buy in bulk and do it via that way as I think they give you money off but I ain't to sure on that one. OC