The link below is to the HUD website. This page will also direct you to the IRS page, US Marshals, US Customs, and more. Each service or branch of the government has their own sales, auctions, and listings. Now with some entities such as HUD you must be a HUD approved broker to submit bids on their homes. So your best bet is to find yourself a buyers agent that is experienced in these areas and work through them. You would also need to be preapproved or if paying cash have proof of funds. You would really need to be ready to go since the good ones do not last long.
Hope this helps,