Hi YZ250H
The main bearings for the crank you can get from a bearing shop, the number is B6206C3 and you need two of these, make sure you get the 'C3' clearance. The oil seals for the crank are 30mm-55mm-10mm and 40mm-55mm-10mm, one of each.
I havnt been able to find out about the gearbox bearings, sorry.
The kickstart oil seal is a 20mm-30mm-4.5mm
The gear shift oil seal is a 12mm-22mm-5mm
The oil seal behind the front spocket is a 32mm-42mm-4mm, but check the depth because a 6mm could well fit too.
I agree with Hoony on the crank oil seals, even though I have given you the sizes it doesnt mean after-market oil seals are as good or made to go in the crankcases.
crank seals Yamaha part numbers are; 93103-30150-00 and 93103-40091-00
Cheers DJ