Purpose of the Application: Allowing people to sign up for seats in our workshops (classes)

Information Displayed for Workshops:  Date, Time, Location, Name/Topic of Workshop

Average Number of Workshops Listed Per Month: 10-15 workshops

Number of Seats Per Workshop: 12

Ideally, the display I have set up would allow clients to filter their search results by location (two different locations) and would then allow a drop down to populate that shows them what classes are available and when. They then can select and register. Once all 12 seats for the workshop are full, it will disappear from the list. 

In order to do this, I'm going to be treating each workshop like it's a product from an inventory, limiting the amount of seats like you would go about limiting the amount of available inventory. This part seems simple to do, but the problem comes up when I want to start filtering the drop downs that are populated. I've had trouble getting the sample look-ups I've worked with to filter based on the criteria of the location of the workshop. 

Does anyone have suggestions on how to make this clean and how to manage the 'inventory' of the workshop seats a bit easier?