All these online stores have the same kind of search system - just type "Pseudo Echo" into their search box (not the Yahoo one below) to get the full listing of what they have.
Sanity Music (Australia) and HMV (Australia) - Both of these currently have Best Adventures and Long Plays. If you're in North America your best bet is good old Amazon (which also sometimes has more obscure and secondhand things in its zShops section).
GEMM - This is the first place to look for secondhand, deleted, rare and hard-to-find stuff, especially on vinyl. An online "marketplace" where a large number of retailers list items for sale, it's a goldmine for collectors! There's always a long listing for Pseudo Echo, so if you're after that picture sleeve single that's been missing from your collection for years, start here!
MusicStack - like Gemm, lots of sellers with copies of nearly everything (this link will give you the list for PE).
eBay - Online auctions aren't everyone's cup of tea, but interesting things do pop up here from time to time. Worth keeping an eye on if you're a collector. A useful feature is mirror sites for different countries so the prices come up in your own currency; click here for Australia and here for the UK. Other countries also available.
You can also see what's
available in the Yahoo Auctions right here!