My fuel pump #3853970 needs to be replaced, where can I find one?

The original OMC number for your fuel pump was 0986417 which was discontinued.  What people used to use was 3853970 from Volvo Penta.
However now 3853970 is also obsolete.

You can use the 826493 fuel pump from Volvo Penta, but you will probably have to change your fuel lines or what most people do is purchase
two adapters to allow the same lines to be used.  I say probably as the change of fuel lines may have already been done in the past.

The other option is to swap to the Mercruiser 1991 3.0 pump 86234A-4 which is obsolete, but Sierra has a 18-7278 which should work.
It looks different, but it will fit and it will probably use the same fuel lines (but not always).