Getting to Silvia from Popayán is not a problem. In fact, the travel time has been by a third from its previous 90 minutes so it only takes an hour to get there now. Direct buses leave frequently with a few plying the route. Though timetables change from time to time, a basic guide is: Tax Belacauca runs at 6, 7 and 10 AM and Sotracauca runs every half hour from 6 AM. The price is 5000 COP ($2.50) for the one hour trip. Alternatively, you can grab a bus to Piendamo and change there for Silvia. This sounds more complex than it is and if you arrive at the terminal when one of these less direct buses is going, they will funnel you on there figuring you wouldn't want to wait for the direct one. Only a gringo would do that and really there isn't any reason to do it as once you get to Piendamo they literally walk you to the next bus, especially if you are a gringo. It actually worked out 1000 COP (50 cents) more expensive for both of us but we didn't have to wait around at the terminal for the direct bus to go. We also got to see a small town that we would not have seen, albeit very briefly.
The buses drop you off right at the main square in Silvia and that is where you get the bus back as well. On the way back we got the direct bus as it was conveniently going when we wanted to leave anyway.
If you are comming from Popayan, it is a strait forward affair. There are direct mini-buses leaving from the Popayan bus station.
Leaving Silvia, there are direct buses to Popayan and Cali. If, as I was, you are heading to Tierradentro or other points east such as Neiva, it may be faster to catch a colective jeep along gravel roads south to Totoro, from where you can catch the next bus heading east. Alternately, for a more comfortable ride, you could backtrack back to Popayan, then catch a bus from there, but that would add a few hours to your travel time.