I am attempting to get a job. (Laughs nervously) However of course I can only find one heel! Can I wear pants and a pair of black tennis shoes? Or do I go out and buy an outfit/shoes then return them after the interview?

I would say it all depends on the job you are interviewing for. If it is an office type / administrative position you might want to try and borrow an out fit from a friend or relative. Don't buy work clothes till you have the pposition and only buy a few sets till your second paycheck at least. If you are going for a retail sales position then that is another type of out fit. Bottom line, unless you have the means don't invest in a wardrobe till you know you have the job and are actually working.

Keep them for future interviews, professional outings and get togethers.

I think you should get a job as a cook! Nobody would probably notice if you wore one heel and one black tennis shoe!