I have some Walnut that's 4.5 x 4.5. It was harvested in the mid sixties, and has excellent figure. I also have some genuine Lignum Vitae that's 6 x 6, but likely prohibitively expensive.
I'm usually able to find large dry stock. It's really a matter of how much are you willing to spend, when it comes to large stuff. Hopefully, you can find something from these folks:
Got Wood? Domestic Wood Turning Blanks - They have large spindle stock from time to time and not real expensive.
Tropical Exotic Hardwoods of Latin America, LLC - Cocobolo & Bocote Specialty Shop - Use the SHOP ONLINE link and then look at:
Turning » Turning Squares » Standard Size Turning Squares » Cocobolo & Bocote Turning Squares
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Turning » Turning Squares » Select Pieces » Bocote
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Turning » Turning Squares » Select Pieces » Cocobolo
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Turning » Turning Squares » Standard Size Turning Squares » Cocobolo & Bocote Turning Squares (5 x 5 x 12 Cocobolo @ $70 - not bad for waht it is)
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Logs/Billets » Cocobolo/Bocote Billets/Logs
https://www.gilmerwood.com/ - The site is huge and you might do better calling and asking for Jim MacDermot (1-888-667-3979). They have so much more than what is on the website and Jim is very helpful. It's not unusual to find Bloodwood and Bubinga in large sizes.
If you're still unable to locate something, let me know. This just scratches the surface...