You can try eBay. You might search for “bonsai tables” or “bonsai display stand”.
The selection may vary.
Then, there are the “bonsai auctions” on Facebook.
And, the larger bonsai shows often have vendors with stands.
David Knittle makes the best stands in the USA. He’s expensive and has a waiting list. You might try “theartofthedaisa”. (Google it). He makes good stands. Sean Smith is known for his stands.
Local clubs often have auctions, and retiring members often sell their stands.
There’s a limited market for bonsai stands. Most people who practice bonsai never show their trees and have no need for stands. So, if you find a good one, get it! You never know if or when you might find another one like it.
Oh... I nearly forgot... the Pros like Bjorn and Ryan Neil might have stands for sale. They will be expensive, however.