A. Hot-fixes are builds of SQL Server just like service-packs. However they are not fully packaged or given the same level of testing as a servicepack. They should only be applied to resolve a particular problem.
To get them, the official answer is that you must contact Microsoft Product Support Services by phone. You can get contact details from http://support.microsoft.com/support/supportnet/default.asp. The call screener answering the phone will charge your account/credit card for the call - in the US this is $195 I believe. Then when the engineer you are put through to determines the call is about an Microsoft bug they will refund it. They will tell you where to get the hot-fix, or will send it to you.
Unofficially, you can pick up hotfixes from ftp.microsoft.com/bussys/sql/transfer as long as you know the build number and pkzip password you need. The files are called SQLbbbm.EXE where bbb is the build number and m is the machine type. So SQL324I.EXE is build 324 for Intel. The files are pkzip password protected.
Wherever you get one from be aware that hot-fixes are typically NOT regression tested and can cause a lot more damage than they can fix. You are strongly urged to discuss using any hot-fix with an Microsoft PSS representative before applying it.