Other mistake: At the beginning of the film when April sees the foot clan braking into the shipping container, it is pouring with rain, yet her hair miraculously remains dry all the way through.
Other mistake: When Donatello is deactivating the countdown, the last 5 seconds on the countdown is shown and the voice on the computer is heard counting down each second. Every one second the voice on the computer counts down is actually two seconds passing in real time.
Other mistake: When the turtles blood is being drained, all four tubes have blood running through them, even though there are only three turtles in the containment boxes.
Other mistake: When April views the tapes of her dad she's around 10-11 in them, but Sachs looks the same as he does in the film and this took place in the past perhaps 15 years, before current times.
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.