I asked on social media if anyone had a spare one in the back of their closet that they wanted to get rid of. A friend gave me a sports-branded one and it's been great for what I need to carry for walks (it doesn't have space for water, though, unless I only want to carry 250ml worth, or a cup).
I have a SPI belt but mine is too small to carry what I need (blood sugar meter, lancet, test strips and 20 jelly beans). So if you get a SPI belt make sure it's big enough to hold your stuff.