If you USA try here for parts
Gasket sets i always use these NE brand ones excellent quality never had a failure
You can Buy full gasket sets here or Top end only here
As for bearings i try to use Koyo or SKF bearings
Athena Seals New oil seals need to be fitted on every rebuild
I always use Vertex pistons only ever had good results with them
I sometime use Mitaka barrels 2 i had good success with them so cant fault them obviously OEM is better .