SSR's are available to switch DC. You just have to look for them. If you're switching less than 3 amps, you can use ODC5's, or the AC output version OAC5's. Both trigger on 5v DC input. These are small SSR's, with pins coming out the bottom, as opposed to the larger 15A or 20A version with screw terminals recommended by the original poster. You can usually find boards full of these on ebay in the "Business and Industrial" category of ebay for almost nothing. They're heavily used in industrial control. If you're really a gadget freak, there are also input modules where they'll switch higher voltage AC (IAC5) or DC (IDC5) down to a trigger-level signal, as well as 12v and 24v input and output versions.