There are four primary ways of obtaining Team System:
Retail - The components of Team System are available from retail outlets (such as Amazon.com). If you are buying the product retail, keep in mind that (a) you are paying full price, and (b) you are not going to benefit from Software Assurance (SA). Software Assurance guarantees that if Microsoft releases any Team System products off band (such as Team Edition for Database Professionals), you will get a license for it at no extra cost. It also applies if something like a "Team Foundation Server R2" gets released; you will be entitled to a copy.
Reseller - Resellers such as SoftChoice provide good value because they have licensing experts that can find and tailor a licensing package to your situation and environment.
MSDN Subscriber Downloads - If you get a combination of Partner, Academic or GSI Programs and own a MSDN subscription, you may also be able to benefit from substantial savings by upgrading to Team System. You can view more information about each option at the following link: http://msdn.microsoft.com/vstudio/howtobuy/.