Sorry you're out luck. The Rc68 is IR remote. Your 3500 stick only takes RF remote, a RF remote that no longer available, not that its sold out they just don't make one that will work with 3500 stick. The 3500 stick isn't worth buying a new remote even it were available. Upgrade to new one with the sales is $22 now for the cheapest roku.
As stated by @Tivoburkee, we no longer offer a replacement remote for the Roku Streaming Stick model 3500. However, we would be happy to see what we can do to help get you back up and running.
Please feel free to send me a PM (private message) with your Roku account email address and the serial number of your Roku device (printed on the side/back of the device) and I'll have our support team reach out directly to assist you further. Meanwhile you can use the free Roku mobile app as a replacement remote control.