Nope just go into a bike shop and ask for them to order in a 57tooth rear sprocket for a dt125 (****) and they will tell you what sizes you can get if they don't have the 57.
You can get any sprocket machined one off to your specs Any bike,number of teeth,pitch and steel or alloy I've had one offs from these guys.Top quality parts http://www.bandcexpress.co.uk/