I got mine on eBay, but it doesn't seem like the manufacturer is selling them any more -- at least, I can't find the listing. Still, if you do a search on "stock" or "stockade" in eBay's Mature Audiences category, you'll find a few others.
E-mail me if you like, and I'll pass on the e-mail address of the folks I bought the stocks from, but no assurances that they're still in the business.