Revealing mistake: When April O'Neil first tries to take pictures of the foot clan activity at the docks, almost every time they show her phone, the screen is black. In fact, when she finally tries to get the shot of the Vigilante we see a clear as day reflection of herself on the screen.
Revealing mistake: At the end of the movie, the Turtles accidentally destroy Vernon's new car. If you look closely at one of the last shots of the movie where you can see the car now upside down, you can see the car doesn't have an engine, presumably to make it easier to flip.
Revealing mistake: When April first takes the photograph of the symbol, she actually has the camera in selfie mode, you can see her face on the phone.
Answer: While never really adressed, the most popular fan theories suggest that either: 1. Splinter manufactured the weapons with materials he found in the sewers or 2. Splinter scavenged for the weapons at a local dump or 3. They were ordered online (only applies to the 2014 version). Fans admit these explanations are kind of a stretch but certain scenes across movies and/or series lend credibility to one explanation or the other. 1. They seem to know how to improve or even craft their weapons meaning that someone taught them (Splinter). 2. They seem to be troubled when a weapon goes missing or breaks meaning they have limited access to spares. 3. Money doesn't seem to be an issue considering all the pizza that is consumed (in some cases the Turtles have been seen holding down jobs like tech support or birthday party mascot). The original purchase could've been made with money scavanged in the sewers and delivered to a P.O. box Source: https://tinyurl.com/y6kbxhlq.