Is there a way to get my Canon Canoscan 8400F scanner to work with OS Mojave?

they don't support it anymore

CanoScan 8400F - Support - Download drivers, software and manuals - Canon Europe (canon-europe.com)

there are third party options but it costs

VueScan reverse-engineered over 6,000 scanner drivers to add macOS Catalina compatibility: Digital Photography Review (dpreview.com)

The CanonScan 8400F is supported by VueScan Professional without any Canon drivers. The current version of VueScan supports macOS Catalina (10.15.*) thru Ventura (13.*), and for Mojave, you will need an older 64-bit version of VueScan. I would check with them about the last version suitable for macOS Mojave (10.14.*).

You are welcome. I didn't mention it but VueScan Professional can be trialed, though every feature on your scanner may not be available until the product is purchased.

I used VueScan with my 8400F CanoScan for several years thru Monterey.