If you're having trouble finding an OEM shift knob, here are some nice options:
Expensive, but very nice and rare 6 speed RX-8 ROTARY SPIRIT shift knob:
Shift knob from a 2016 Miata:
2016 Mazda MX-5 Shift Knob - The News Wheel
I've also attached two more pictures of some Honda Type R style shift knobs. I have used one of those before in a Mitsubishi Lancer and found that it really helped improve shifter feel and made the shift throws feel positively shorter and more notchy. Would be a great addition to Mazda's already great manual transmission if you really enjoy driving manual. Just throwing some ideas at you.