This time it is a two-part question about stitching supplies.
Where do you buy floss and fabric?
This answer is a little involved. I have so much floss that I've been collecting over the years, I don't have to buy much. If I'm starting a new pattern, I hit up my stash for the required floss. Any colors that are going to require more than one skein, I will buy new skeins so that I know I have enough of the same dye lot. I do have a local quilt store that stocks DMC so if I need only a few skeins, I'll get them there. Unfortunately, it's expensive there, about 69¢ each. If I need more than say, 10 skeins, I'll buy them online.
When my local LNS went out of business, I worked her going-out-of-business sale in exchange for stash. I got a lot of fabric! I'm pretty set for 28 and 32 counts.
If I do need floss or fabric, I shop primarily at 123stitch. If I'm doing something like a band sampler where the fabric is visible, I like using dyed fabrics and Silkweaver is my favorite. Both these companies are in the US. For online shopping in the UK, I've heard a lot of good things about Cross Stitch Heaven.
Leigh also asked:
I've also never stitched on linen before, so I might try to do a smaller linen project first as a test run. Any tips for switching from aida?
Yes. Run away from aida. Sorry, just being a fabric snob. ;-)
If you've only stitched on Aida, I wouldn't suggest linen. Linen can be great but it tends to have variations in the thickness of the threads. It also costs more than evenweaves. What I would recommend is either laguna or jobelan evenweaves. The best tip I can give is the starting point. This is very well described and illustrated at Kathy Dyer's site, point #18, so I'll have you check it out there.