Hi Kelly,
Changing your posting account defaults is indeed the answer.
We can't tell if you are using the Fixed Assets module, but here are some pointers on where to change the accounts if you are not using Fixed Assets.
If you only want this to apply to purchases from a specific vendor, go to that vendor's card, click on the accounts button on the bottom of the card and change the accounts there.
If you want it to be a company default, go to the Administration area page | Setup | Posting | Posting Accounts, select the Purchasing series and change the accounts on that window.
Kind regards,
Leslie