Can I be fired for not being able to purchase non-slip shoes?

If the person that interviewed you was given the authority to interview you and to make decisions about your being hired and your employment - the non-slip shoes is a part of the employment - then you should be able to rely on their authority to give you an extension to purchase the shoes.  You need to sit down with whom ever is pressuring you and explain that this was a condition of employment on your side because you explained before you were hired that you sis not have the ability to purchase the shoes and were given an extension of time to do so.  That the person had all the indicia of being able to make these decisions along with the hiring process.  Maybe they can advance you the money and you can pay it back little by little each paycheck as  compromise.  Get it in writing.  Good luck.