Yeah, I was thinking of the pockets I used when I used to do photography, had some for negatives, pretty sure I had some for 35mm slides as well. Definitely had some for medium format slides/negs. Happy days toting my Yashica TLR around...
But anyway, I found some 2ml centrifuge tubes, 500 for £11.24. Works out as 2.248 pence per tube - don't think I'll find anything cheaper. I've been saving closed cell foam packaging, just have to drill lots of holes in that to store them.