Correction: get any old WiFi router. Most older routers only had ethernet, not WiFi.
In any case, if you get a WiFi router (or just a WiFi network hub if you already have a working router), you can configure it to just about any setting you need (from the worthless WEP encryption for the DS, to the mostly secure WPA2 encryption nearly everybody uses).
FYI, Nintendo stopped selling/supporting their WiFi dongle because of the licensing issues they had with the manufacturer, BroadCom. BroadCom are the most unreliable manufacturer in the WiFi industry anyway, so it's no big loss. If you really needed a USB dongle to create an Access Point/Hotspot, just about any will work if you configure it properly (in other words use the provided software to set it up as an AP then use Internet Connection Sharing to share the internet your comp/laptop already has).
Another solution, if you have one that is, is to use a smartphone as a portable Hotspot. It'll only give 3G internet and not full speed broadband, but that should be good enough.