I assume you mean by switching positive that the SSR turns on when it receives a high signal. I assume your controller sends a logic low signal.
If so a simple transistor switch will work. Vin is the link from your controller and VO is the link to the SSR. A PNP version would work if I happen to have the relative 'polarities' the wrong way! The transistor wouldn't be critical and any gen purpose NPN (PNP) would work.
The ULN2803 IC should also work. Depending on the switched voltages a 2804 could be better. A ULN circuit would be more simple than the transistor.
Looking at the SSR, I take it you want to drive AC elements at 120-240V? If it is standard LED elements @low DC voltages you probably wont need the SSR.
A typical NPN transistor such as a BC337 should work. They are very cheap (~10cents) and should switch more than enough current for the job and handle C-E voltages up to 40V. I would look to use collector and base resistor values of between 1K and 10K. If you're using 12v power supplies then 10K will give 1.2mA current. That should turn the SSR on.
The DC controller does indeed pull it's output low when on. This is why you need a PNP transistor.
In the idle (off) state, the controller output is floating. The 10K resistor from the PNP base to emitter (+5V) keeps the base also at +5V and the transistor stays off (collector effectively floating). This means that there will be no voltage on the SSR input, so the SSR will be off.
When the DC controller channel turns on (pulls its output to 0V), the base of the PNP transistor will be pulled (via the other 10K resistor) 0.7V lower than its (+5V) emitter and turn on. The collector of the PNP transistor (going to the SSR input) will now be near +5V and the SSR channel will turn on.