It was just an ordinary van that belonged to Baxter Stockman (who, both along with the van appeared for the first time in "A Thing About Rats"). After defeating him and delivering him to the police, the Turtles take his van to drive to Shredder's Mousers factory (and destroy it). In the next episode "Hot Rodding Teenagers from Dimension X", Donatello rebuilds Stockman's van into The TurtleVan, equipping with all kinds of technology the Turtles found in an abandoned factory, in order to find the Technodrome easily and fast.