Do a google search in your area for "spy shop" and see what you get. I've seen retail stores around that specialize in surveillance and counter-surveillance tech, supposedly for businesses that need to have good "OpSec." But they usally also cater to individuals who are looking for personal safety from an ex or who might be trying to escape an SO for "off the clock" activities.  

 
Or, just have an honest and open conversation with the SO - AFTER you hire a good attorney. :p  

 
Life is good

 
The Cat

with my first wife is that once they suspect, they employ many tracking strategies.  Overcoming one might not be enough.

Your goose is probably about to be cooked.  

It's an interesting question so I looked around online out of curiosity.  I didn't make much of an effort, so there's likely a lot of other ideas out there.  However, GPS spoofing seems like it could be the thing for you.

I found one product.  I'm sure there are others.  Good luck or fess up!

If your phone is an Apple iPhone, note that you can choose which of your iOS devices is the one tracked (and the trackER isn't shown which one it is).  So if you have another iOS device, maybe an iPad Mini or a cheap older iPhone, you can add it to  your AppleID account and then set it as the tracked device.  That's only the beginnings of a solution, of course; you have to put that device in believable but non-incriminating locations while you escape, but it could help.  

If the goose is already in the oven, though, it probably wouldn't be enough.

As the others say, no solution is going to be perfect if the SO is suspicious. But this could be a good case for a burner phone and/or a Google Voice number.
You could leave your main phone in your office (or somewhere else) and just use the burner. I think your can adapt Google Voice numbers for both, but haven't tried it.

...although most people don't seem to believe it's real.  Discuss polyamory with your SO.  Or ENM (ethical non monogamy)  Or an open relationship.  Read books on the subject, read up online, attend meetings of the Lovingmore organization or visit their website.  Discover that having a solid relationship at home with zero deceit, drama or jealousy is actually possible, and make that a goal you and your spouse work towards.  The benefits are so worth it!