If you just need a few or several assessments on a one-time or infrequent basis, just add them to your cart and check out. The order email will contain a unique link for each assessment purchased, which can then be distributed to the respondents. You can also find those links in My Orders.
If you are looking to purchase assessments on an ongoing basis, you may be better served by setting up an administration account. Learn more about EPIC Accounts to see if this is a good option for you.