À la carte graduate courses
I believe the term you might be looking for is a "non-degree seeking student". Many colleges - at least in the US - allow students to pay per credit-hour of course they take, with the explicit arrangement that they will not count towards a degree. Exact requirements, costs, and arrangements vary by University, but here is some info about the options at Michigan for example. Michigan's School of Education has its own page on this, with additional options.

The 'catch' is mostly money, whether or not you can get classes that interest you (Universities might reserve some classes for their own degree-seeking students first), and whether or not courses taken in such a way will count according to your employer (you'll have to check with them on that). 

As for the money, non-degree seeking students generally receive no form of financial aid or tuition assistance, and in the US I would be surprised if you could take even 1 course for less than $1000, but the exact cost will vary wildly by University and program.