I don't think you need a spiked collar, as long as your tree has a single trunk and no close-by tree for a cat to jump from. Just wrap a collar of CLEAR polythene about three feet deep round the trunk and the cat simply skids back down when it tries to climb. It worked a treat below our nest box last year. But the polythene needs to let light through or it won't do the bark any good.
If a cat can't dig its claws into something like bark, it can't climb.

Don't know if it would work but have you tried that plastic spiked stuff that's sold for the tops of gates? Probably not cat proof though...

Hi, wondered if anyone might know where I could get a spiked tree collar? Trying to do what I can to protect our nestboxes and prevent the local cats from getting up to them.

Cheers,
Tom