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.
Continuity mistake: After the turtles, April and Vernon escape from the Sacks compound in the 18 wheeler, the truck then get stuck in the snow. When they realize they start to slide backward down the hill, Vernon guns it. But then they show the wheels spinning on the Freightliner Argosy Cabover with a single axle. Next shot goes to inside the cab and then back to outside the truck, and it's back to a dual axle truck.
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.