seems to be a few options for a speed/cadence sensor for a garmin 810 even within garmin kit itself from an all in one to a two piece job.

can anyone recommend any?

Much of a much-ness really. I have a bontrager dual one. It works. simples.

Only gripe is that the rubber clippy thing can pop off, so rather than using that, I use zip ties, so its not really something you can jump between bikes much.

The new magnetless Garmin sensors a fine. If funds are tight forget the speed sensor.