They're easy to find under brand names like Kalloy, Zoom, and Dimension using keywords adjustable+quill+stem. It's a good idea to use an adjustable stem to dial in your fit and they're inexpensive. Make sure the quill and bar-clamp are the correct size for your application. Quill stem risers can be short or tall but all of them require at least 65mm of minimum insertion. Here's a good stem calculator tool: Stem Comparison Tool | yojimg.net