I've found flickr to be a useful tool over the years in connecting with other collectors of various things (80s/90s toys, vintage stationery, etc). I don't know about a specific Sweetie Pups group, since my main use for flickr is in cataloging my vintage Japanese stationery and toy collections. But the connections I've made there have helped immensely in hunting down some rarer items I was in search of. There might be some vintage toy collectors on there that might be able to help in your search? I know there are a lot of vintage toy groups on there. You can't actually buy/sell there since it is against the rules (although there are still people who do it) but connections with other collectors can be really useful. I've seen several people on there who have/collect prototypes of vintage toys.
I don't use it much, but I know some people use instagram for the same sort of thing. Posting a "In Search Of" picture of what you're looking for and connecting with people who collect similar things might be a good idea.
You may already be doing this since you are an experienced collector, but using the "save search" feature on eBay is a great tool for being alerted to new listings of items you are in search of (they get emailed to you as well as posted on your front page). Saving different variations increases your chance of the system picking up listings you might be interested in.
Sorry I can't give a more helpful answer!