You can get the USB-A to USB-A cable from some specialist computer stores, but this is not a common cable.
I wouldn't recommend connecting two computers in this way, mainly because you'll eat up ALL the bandwidth leaving nothing behind for use by your printer, scanner, keyboard, mouse, and any other devices that may be run off USB.
The best way of connecting two computers would be to use ethernet cables like 99.99% all networked computers.
I recommend finding a networking kit for your systems, almost all the manufacturers have on e or more, Netgear, D-Link, Linsys, etc. This is what they normally contain:
1 HUB or Cable/DSL Router
2 CAT5E Cables
This will give you more expandability, and it will allow you to share the broadband service easier. I am set up that way, and I have 3 systems on the HUB, and my Notebook can connected whenever I need to, and they all share the internet.
Plus if you use a router you don't have to have both computers turned on all the time. Just leave the router and modem on and connected so you keep the same IP address if the ISP uses dynamic addresses and so your conenction is always valid when you power up the either PC.
I've seen 4 port routers as low as $20 after rebate at best buy in the last month. Shop around.
If the second PC doesn't have network connection on the MB a NIC card is $10 or less now.
The cables you'll need should not be more than $5 bucks. Do not let somebody rip you off charging $10-20. It infuriates me when I see stores pulling that crap!!!!!!! Compusa is the worst.
PS Linksys rocks (been using mine for 5 years now). But netgear has one that may be better now. It's the streamlined looking one.
Solved connection dropout problems when working through an ISPs VPN system. No other router worked for the guy. He was using the BEFSR41 with every new firmware version that came out. When he went to the Netgear RP614 his connection became rock solid again.
Warning. Some of the netgear firmware upgarde files on their website have the wrong extension on them and will not load properly. You have to change the file extension before flashing. Lame, I know...