I also have lots of goldfinches, I feed them on Sunflower Hearts. I buy them from a firm called Titmuss and Son, any idea the cheapest place to buy them on the internet?
I have tried many others as we get through about 25 kilos a month but Titmuss always seems to be cheapest and consistent quality bakery grade seeds. Used to use Kennedys till I found out they get theirs from Titmuss anyway. You may save a few pennies scouring ebay for a few hours and working out the actual cost per kilo to compare them when postage is added but in my experience it is usually very poor standard dusty grain that really clogs up in the bottom of tube feeders
I've just received sunflower hearts (next day delivery) from Natures Grub - 2 bags of 3kg at £5.11 each. It's a useful size if you haven't got much room for storage or, like me, are trying to keep to a budget.
http://shop.naturesgrub.co.uk
We purchase our sunflower hearts by the sack full as the birds in our garden go through them like wild fire. We get them from a local supplier in Hampshire at £20 for a 13kg sack.
We have a lot of goldfinches in our garden and in addition to sunflower hearts they are also partial to nyjer seeds.
http://www.flickr.com/photos/all_things_nautical/