I have seen these recently, last 2 weeks, in Swallows Aquatics in Raleigh 5 for £5.50 and in Wet Pets Victoria Road Romford (slightly dearer if I remember correctly).
Both of these are relatively near depending on what transport you have available.
i've only seen shrimp a few times in there, and he tends to be a bit expensive.
i've finally got round to setting up the 10g tank i brought from you, it's now my shrimp tank!