Do you want fries with that nunchaku?
Even with the dark web readily available with your computer, you wouldn’t have an easy time trying to get ninja weapons. And the turtles lived in the 90s so there couldn’t have been an online ninja store! Sure, they would certainly pick up whichever weapons their adversaries left behind them, but those wouldn’t have included ninja attire! So where did Raph get his sai, or what about Mikey’s nunchakus? Are they friends with some smugglers?