Yo, welcome to the forums! I've already seen some of your posts around here
Being a big time cheapskate myself, I quickly learned that all the junkyards in my area don't inventory many of their shift knobs and you can often get them for free. Downside is that many of the good ones are gone. While this might not be the case for you, it's something to check on.
After I bought my P5, I realized that the stock shift knob was kinda lame, so I went looking for OEM knobs. I eventually found a weighted Mazda 3 5spd knob which, while not fancy, was in good condition and gives a nice, modern OEM look [see attached image]. I also found a Mazdaspeeed3 leather boot with red stitching, which I also got for free and installed with some modification and cleaning.
In terms of aftermarket, you can find something cheap like this:
Generic shift knob - eBay
I got one of these with my Megan Racing short shifter kit and it's not terrible, but not the best quality and kind of generic.
I've seen that many nice, OEM shift knobs on eBay (Miata, RX8, RX7, etc.) are kind of spendy for what they actually are, and I personally avoid them. Not sure what your budget looks like but that's my preference.
Best of luck!
BenjiHoggi