Ooh Janice, this is a hard one!
Just about the only cardstock available in the UK (12x12) is Bazill - all 3 shops I use stock it.
You really need a cardstock swatch whcih they all sell as the colours are different on the internet. It is only available by the sheet and costs between 20p - 50p depending on the colour and which shop. The price varys a lot between shops with ScrapMagic being the most expensive I think (I know Fiona noticed a big difference, I was just in a buying frenzy and never even looked at the prices...)
What colours -I buy bits and pieces, I have no coherent system although I wish I did. The only thing I do do is make sure I buy at least 2 pieces of each colour, so I can do a double spread if I want to.
If you can get to the crop then you can pick colours to suit the photo's / accessories you are using and it is much easier. For at home I just periodically order a selection of colours. In fact, I am in the middle of an order on my other session right now.
If you are in the States then packs of card are available in Michaels - I found 12x12 but I think Fiona didn't - they should also be available at other scrapbooking stores in Orlando - even
Walmart has a little section. They are much more economical and come in a mix of colours - like 'Country' or 'Beach' so all the colours should work together.
I use all different colours and styles in my album - but I quite fancy doing a small album with just a few colours/papers that is co-ordinated thoughout. Maybe one day!
I buy mine from Artbase or Scraptastic (I usually get card from Scraptastic, but no idea why) and it comes in a pizza box. I don't know what the shipping is - I think Scraptastic charge the actual shipping (which would be nothing for you) and Artbase have a fixed rate. I told you I don't worry about the cost, I just buy and cross my fingers when they ring it all up!
Now, patterned paper is a whole different thing -I love it, it is so pretty and unusual, so I buy what ever I fall in love with - until recently I never used it, just bought it and put it in my folder, but I am now making a concerted effort to use it. Patterned paper can cost anything from 50p - £1 or more a sheet so I do try and check the price before I pick that up. I never used to buy 2 sheets because it was expensive, but I am now finding it is hard to work with only 1 sheet so I am trying to remember to buy 2.
I also like Vellum which is a see-thru paper a bit like wax paper - I just buy a pack of 8.5x11 and I use it for loads of things. It's nice for typing journalling on.
everything to do with scrapbooking costs a lot!