Good cheap place to buy veg seeds online?

Elsewhere

I've never had any luck with that, but my soil is dreadful for potatoes - thick clay.

As a child our garden had fairly light soil and all sorts of unexpected things would pop up - certainly potatoes from stuff that had been tipped on the compost heap, occasionally tomato plants (I had a tortoise so bits of tomato were regularly left in the garden). I know of someone who successfully grows peppers from the seeds he gets from supermarket vegetables.

I've tried to grow black squashes from the seeds of one that I was once given; I got some huge plants but sadly they didn't set fruit.