Although your needing alot... search ebay.

For example...

https://www.ebay.com/itm/20-Qty-Extruded-U-Nut-5-16-18-Screw-Size-J-nut-/162574952874?hash=item25da38e9aa:g:EC0AAOSw5cNYSa9c&vxp=mtr

Here is like 200 of them...

https://www.ebay.com/itm/200-BULK-Qty-Extruded-U-Nut-5-16-18-Screw-Size-8372-/112581211942?hash=item1a365cf726:g:cioAAOSwNRdX~p1c&vxp=mtr

Many pages to look for better deals.

non ebay....

http://sourceautobodyfasteners.com/index.php?route=product/product&product_id=435