Here are a couple of links to Lignum Vitae being sold on e-bay uk.
I also have some “Vera” wood which has very similar properties. Ebony is also very tough and good for a pivot block but that is getting expensive, odd ebony piano keys come up on ebay from time to time.
I just bought a couple of pen blanks approx. 1″x1″X5″ on eBay.
About how big are the slips that you use? I figure I can get each one of those little planks ripped into thirds.