Eventually, you reach a point where the picture is no longer visible and all you have are black dots. Finally, remember you are looking though the Earth's atmosphere and that will ultimately control the limit of magnification you can use. Dirty and turbulent air will restrict your magnification much more than clean and stable air will. No matter how good of a telescope you're using, you have to look through the eyepiece. If you have lousy eyepieces your image will be lousy.
Eyepieces are relatively cheap.
Spend the money to get high quality eyepieces. The difference will be worth it, and if you take care of them they will last forever. There is a nearly limitless number of accessories you can buy for your telescope. I suggest you work with your telescope and become familiar with it before investing in additional accessories. It is possible to spend as much money as you care to, so be sure to set yourself a budget.