Besides Internet Auction Sites like EBay and Ebid,you could try Train Shows and Swap Meets.
Also,some Hobby Shops might still have them in stock.
]Depending on the road,your dealer might still have a few in stock.
While some roads that are popular like SP;UP and ATSF might sell out quickly,less popular roads like MP;Katy;Frisco and NdeM generally aren't big sellers.
Also,check out the For Sale and Trade Ads in the back of MR/RMC/MRN.They might have what your'e looking for.You also could try putting an ad in the same mags as well.
Finally,join the HO Yard Sale Group on Yahoo.I know theyr'e mainly HO but someone on there might be able to help you.