Add a Yes/No Field to your Table (if you haven't already) and add criteria to this Field in your Query.
Base your Combo Box off of the Query you just built.

I made an image as a visual representation.

First don't be afraid of a macro or a little bit of VBA.  Think of it this way:  Access doesn't "automatically" know what you want to do.  You as the database developer have to give Access the instructions on what to do.  So "Programming" is really just writing down, in a language that Access understands, those instructions.  You don't have to learn everything at once, just a bit at a time.

Second,  I'm going to highly recommend that you consider a TechHelp membership.  For about the price of a Starbucks per MONTH you get a ton of resources you don't get in the regular courses. The best in my opinion is the Gold because of the perks such as the Code Vault.  Even more savings if you get the annual - save 20% more.
More info in check out the TechHelp Memberships page.

The Manage Rental Inventory video shows how to set up a simple check in /out system.  The extended cut is really what you want to be looking at.

Check out the Query Criteria video to use as the data source for your combo box as well.
You will want a field in your equipment table "CheckedOut" - Yes/No datatype  to limit the data of the Relational Combo.

Kevin thanks! I think I went on such a design bender overnight that my brain was just fried. That does seem like a pretty simple solution. I'm having a few buggy issues on my continuous form that causes it to make a few errors (the combo box isn't updating consistently) but I added a 'requery' command on the fields upon clicking and that seems to have really tightened everything up. Thanks again!