You could go and pick your own, they're growing everywhere at the moment.
Although this doesn't talk about vitamin C and echinacea combined, it pretty much discredits using either individually to cure colds
www.nhs.uk/livewell/coldsandflu/pages/preventionandcure.aspx
( my other username is a porsche , posting shit pictures so you don't have to, Tue 22 Sep 2009, 7:45, archived)

I can't find the original papers, just the news articles referring to them
They were in some journal at my uni. However, the NHS is right; taking them daily BEFORE the cold has taken effect is obviously more beneficial, as it doesn't fight the virus, it boosts the immune system itself.

HOWEVER there are also studies that say taking echinacea for more than a set amount of time reduces the effects of it anyway. I found as many studies showing this as I did showing that MMR caused autism. Which is very few.
( Methylene Blue - electrohead, Tue 22 Sep 2009, 7:54, archived)

Sorry, Cochrane have dealt with vitamin C and echinacea.
Vitamin C is pretty conclusively found to have no benefit for prevention or treatment.
Also no evidence for Echinacea preventing colds, but apparently some evidence for effectiveness as a treatment.
edit: links www.cochrane.org/reviews/en/ab000980.html
www.cochrane.org/reviews/en/ab000530.html
( my other username is a porsche , posting shit pictures so you don't have to, Tue 22 Sep 2009, 8:03, archived)