Unfortunately, they don't really have garden centres near me as I live right in the centre of London (just near Brick Lane / Tower of London). I love where I live but that's the downside...people don't really have gardens and they don't really have pets.
You can get a great cocktail at 4am, have a meal, see a band, go dancing or to a gallery....but hamster with sand bath requirements aren't so well catered for.
But....whenever I do see a garden centre (or pet shop) anywhere on my travels, I always stop and go in to see if they have anything suitable for my hams.