So here are the places where I get my card making supplies. I try to source shops that sell items that are sufficient quality for card making, at reasonable prices. I hope this list is helpful. Feel free to share other shops you use in the comments.
Card blanks & EnvelopesWhen I first got into card making many years ago a friend said to me "never pay more than 10p for a card blank plus envelope". Craftwork cards have a great back2basics range of plain card blanks, £5 for 50.
Coloured card & Double sided tapeGreat quality card starting at 10 sheets for £1. And by far the cheapest double sided tape I have seen - I got 240m in my last order ! You often pay a fortune for a small roll of tape in shops, and with using over 20m a week on average I needed to find somewhere that sold it in larger quantities at a much more affordable price.
A slightly random find, but Wilkinson sells cello card bags at very reasonable prices. One of the advantages of living in the north :-)
Stickers & EmbellishmentsMy mother-in-law told me about The Range. A similar shop to Wilkinson, but they have a huge craft department. You can get pretty much anything here, but they sell a a good variety of peel offs, stickers and embellishments.
Ink & Fimo
Rather bizarrely, this site is the best I've found for both ink and fimo. They have a huge range of colours of both.
Ribbon can be very expensive, especially if you pay per metre. This site is excellent value, starting at 99p for 40m. It might not be quite up to dressmaking standards, but it is more than good enough for cards.