How can I get tonsil stones after having my tonsils removed?

It's not the tonsil. The tonsils create tonsil stones or tonsilths because of the crypts in them that trap bacteria and food (see my other taps). If you continue to get tonsil stones you may have residual tonsil tissue (possible but unlikely) but you can also get tonsil stones or tonsilths in the lingual tonsil which is at the very back part of your tongue. Debris also collects in the vallecula - above voice box.
Tonsil stones. You won't. They should be gone. Have your ENT doctor evaluate you if you think you have persistent problems.