Where can I find the best place to replace my sprockets and chain with a 4.00 for bottom end speed (12/48, 13/52, 14/56, 15/60, 16,64 combinations)
Thanks

Greetings from Tennessee 
 We need some more info Make,model,and year etc.

Greeting from NZ. So any combo divided by four.
Look forward to hearing more about your bike, model etc.
I'll guess DT1 "because i like DT1's & have one"--a very rough one.
Look forward to your return. Cheers.
ps, this link looks useful & might be good no mater what your bike is.
www.denniskirk.com/sprocket-and-pulley/1...a-dt1-enduro-250.mmy

Its a 1969 250 dt1b, I have a 15/45 on it at the moment but its to fast for the woods can get the front but harder to find the rear.

PBI (located in Oregon), Sprocket Specialists (Utah), and Rebel Gears (Tennessee), are three companies that come to mind that have or will make you almost any sprocket you want.  You may need to call them as gearing changes for 50+ year old bikes might not exist in their current catalog.  I've used all three companies.  Sprocket Specialists seemed to be the most knowledgeable.  All are high quality. 

Yamaha tended to use the same spline and bolt pattern for multiple years/models though this is not always well documented and catalogs for older models are sparse.  JT sprockets catalog has a lot of engineering diagrams that show bolt circles and pilot holes and you might find what you need there.

Why change both, just go down on the front to a 13 or 12.
Here's a 12
www.denniskirk.com/jt-sprockets/self-cle...12n2.prd/8099312.sku