Need help finding seat mount plug screw.

[D-n-Tyke]

Truck we bought was originally a bucket seat truck, unfortunately PO pulled the buckets before we bought it. I found a super clean bucket seat we are going to put in and run for now. I thought the bench bolted directly in the same mounting holes as the buckets but found that not to be true. There are different holes for each seat type, with one set being plugged with large Philips head screws. Going from bench to bucket I would have extra Philips "plug" screws.But going from bucket to bench I am short. I can't find them listed on any of the normal truck supply catalogs, plus I hoped I could just get them from a fastener house maybe cheaper.

Screws a 5/16-18 x 3/4 lg with an extra large diameter (0.685") philips head.
Where can I get these?