A sailmaker would typically describe an asymm in one of two ways...
RIG dimensions
"hoist" (the "I" dimension in a sail plan, looks like 50.0' in the E38 docs on the download page)
"J" (the "J" dimension in a sail plan, looks like 16.25' for the E38)
(** be sure to verify these for YOUR boat)
or SAIL dimensions
luff
foot
leech
In both cases, for an asymm they'll also talk about
"weight" - the weight of the cloth used
"girth" - expressed as a percentage of the J dimension
-- 165% is a common size for a general-purpose asymm
-- 180% is a big powerful sail, usually for racing
"shape" - in general
-- an "A0" is a light-air runner
-- an "A1" is a light-air reacher
-- an "A2" is an all-purpose runner
-- an "A3" is an all-purpose reacher
-- an "A4" is a heavy-air runner
-- an "A5" is a heavy-air reacher
So... if you're looking at a used sail, you might pay attention to the ones described as "165% A2, 50' hoist, 16' J"
The same sail described in sail dimensions would be something like "52.5' luff, 26.8' girth, all-purpose runner"
Note that many sailmakers have "cruising spinnaker" designs - brand names include the UK "flasher", the Ullman "blue line" the North "g-series". These "cruising" sails tend to be compromise shapes made out of heavier cloth in the belief that cruisers will put it up and leave it up when conditions change. That may or may not be what you're looking for. In my opinion (?), if you're looking for a sail that will be usable in a broad range of conditions, a "cruising spinnaker" might work well for you. If you're looking for a sail that will perform well in specific conditions (e.g., running in light air), a cruising spinnaker may not be an optimal choice.
hope that helps....
bruce
Edited to add: there are lots of places to find used spinnakers. You could google "used asymmetric spinnaker" and you'll see listings from a variety of sailmakers (pineapple, doyle, others), ebay listings, etc.
here's a couple of examples...
http://www.pineapplesails.com/usedsail/web_spin.htm
http://www.minneysyachtsurplus.com/spinnakers.html
http://usedsails.com/cgi-bin/webdat...pinnakers&RANGE=&pagenum=3&cgifunction=Search
http://www.ebay.com/sch/i.html?_fro...ic+.TRS0&_nkw=asymmetrical+spinnaker&_sacat=0