Hi Jovaro
Its the 'Husband of Tuppence' here. Tuppence is sound asleep getting a well-earned rest. She has been working very hard and under a lot of pressure over the last 8 weeks or so.
As for your question, yes, there is a source for the virtual particles. You will recall that Tuppence pointed out that Einstein's equation meant that matter and energy were inter-convertable. The energy that is being turned into matter in this case is the Zero Point Energy, which is intrinsic to the vacuum. This energy allows the local manifestation of virtual particle pairs in the vacuum, like electron-positron pairs or positive and negative pions. In fact there is a whole zoo of virtual particles in the vacuum. These virtual particle pairs briefly flit into and then out of existence.
The number of virtual particle pairs of various types fliting in and out of existence in a cubic centimetre at any given time is immense. But that number is dependent upon the strength of the ZPE. As the strength of the ZPE increases, there is more intrinsic energy per cubic centimetre in the vacuum. Therefore the number of virtual particles per cubic centimetre also increases. As the number of virtual particles per cubic centimetre increases, so, too, does the number of hits on virtual partilces by photons of light as they travel through the vacuum. The more hits, the slower light appears to travel over a given distance. Its rather like a runner going over hurdles; the more hurdles, the slower the runner's time over a fixed distance.
In anticipation of the next question you may ask, there is a good scientific reason why the strength of the ZPE builds up with time. It is discussed fully towards the close of the Setterfield/Dzimano article published by the Journal of Theoretics entitled "The Redshift and the Zero Point Energy", which has already been linked by Tuppence in an earlier post. If you need some guidance in getting through it, I think we probably understand enough of it to give you some help.