You can't mail order an atomic bomb

When I was at the NIST IBE workshop, I happened to notice the NIST standard reference material (SRM) catalog. Leafing through it, I noticed you could order a bunch of cool stuff, like, for instance, Plutonium-239. So, interesting question: could you order enough fissile material to make a bomb? Unsurprisingly, the answer appears to be "no." NIST's Pu-239 SRM is 220 Bq and costs you about $1000 ($937). Plutonium's activity is about 4.4x10^{-10} g/Bq, so the SRM consists of about 10^{-7} g. Critical mass of plutonium is about 10 kg (10^4 grams), so you would need need 10^{11} samples, for a price of $10^{14} (100 trillion). Seems a bit spendy. I should probably mention that the samples are 5 ml each, so this would be 5x10^8 liters of fluid, which might be a bit much to manage. Also, I suspect that NIST would notice when you called to place your order.