If you're talking about the stock Toyota/Denso sensor, it might be easier to make one. That's what I did. Take a stainless steel nut of appropriate I.D., grind the hex off until it is smooth and nice-looking on the outside, then cut a flange out of 304 stainless steel (3/16" thickness) to match the O2 sensor flange, drill and tap the holes for the retaining bolts, and weld the nut/body and fabricated flange together, then weld it onto the DP. Mine looks factory, but that's how I made it.
For wideband flanges you can go to a good hardware or fastener store and buy a stainless steel M18 X 1.5 thread pitch nut, grind the hex off of it so it looks nice, then weld it on. Or you can order one from any number of places. The ones that FJO supplies are not stainless, but you can buy stainless ones by K&N (PN 85-21688) or Painless (PN 60406). Jegs used to keep these, but I'm not sure if they still do.