I can't believe this issue still exists in VS2015. We have a solution explorer folder tree. Why is "Current Selected Folder" not an option in the drop down. The "..." doesn't default to starting in the solution folder, I have to browse across the network to t the share, then drill down through all the folders to get at a folder that is ALREADY right there and selected in the solution explorer. Dreamweaver has had this functionality for its search for at least a decade (maybe longer).
While the Choose Search Folders works, it could be much better. I would really like to choose folders to exclude as well as include because sometimes folders are nested. You may want a folder but only some of the subfolders.
But by far the worst inconvenience is that it is not stored with the solution. If I go to the trouble of defining a search, and close VS, then reopen, I have to define it all over.
As said in other answers, there is no immediate way to limit your search to a given folder within the current solution. (There are at least 3 feature requests for this in visualstudio.uservoice.com that have all been closed for lack of upvotes.)
But here is the fastest workaround that I could find.
Works without leaving VS and with keystrokes only (no mouse action required):
In Solution Explorer select the folder to search within.
Press Ctrl-C (copies folder path to clipboard).
Press Ctrl-Shift-F to open "Find in Files".
Enter your search term, then press Tab to forward the cursor to the "Look in" field.
Press Ctrl-V to paste in the folder path from clipboard.
2020-Update: Actually you can select the folder (also the current directory with two clicks) and define file types in Microsoft Visual Studio Professional 2019, Version 16.7.5