I'm not sure what you mean by the tier list thread, but the 'how much is this pet worth' is located here;
Forum/viewtopic.php?f=20&t=4460390You can use the search function to search within a specific board (probably CS Discussion assuming it some type of worth/value thread), I'd suggest trying that with keywords you know will be in the thread. If you know part of the thread title you can select 'search within: topic titles only' to make it easier to find what you are looking for. I also find it helpful to select 'display results as: topics' so it shows a list of topics rather than posts;
Forum/search.php