Where they are?
For airports in the USA, you can make your own RNAV SID files for use in the Route Manager. Its a bit of work to make a SID/STAR file that route manager will fly for you, but maybe worth it if you don't have another way to get SIDS or STARS.
Skyvector.com has all of the PDF files available for the SIDS and STARS for USA airports. You can view or download these PDF procedures.
On each procedure you can see all the GPS waypoints for the procedure.
You can then enter the waypoints in the flight plan route box inside Skyvector's 'flight plan' dialog box.
Skyvector will then plot the waypoints of the SID or STAR and put them on the map.
You can then save the waypoints in a .gpx file that the Flightgear Route Manager knows how to read.
Then when you fly and want to use that sid or star, you can tell Route Manager to load it, and your autopilot will fly it for you.
Notes:
1. These SID/ STAR files by default give you only lateral guidance. You will have to modify them to get altitude control or you will have to manage your own climb/descent rate as the autopilot flies the procedure for you.
2. This is not a good solution for pilots who want all sids and stars available wherever they fly. It's better for those who want to study and fly all prodedures at a given airport or two, for example. For those who want all sids and stars available on demand, the Navigraph solution already mentioned is the only solution known to me.
After you practice a bit, it takes only 2 or 3 minutes to make a SID or STAR .gpx file in this way.
You can make an entire SID/STAR .gpx set for a mid sized airport in about a half hour.
Hope this is helpful,
Simworld2020