Pretty easy to grow your own, from seed even, but most plant nurseries will have these in their herb section.
That person did some lab testing and found high levels of lead in their backyard-grown chard.
About that...
> In addition, the limited testing the FDA has done on spices has been focused on harmful bacteria, such as salmonella, not heavy metals, Ronholm says.
To be really certain you'd have to test your soil.
I don't think you could test seeds without destroying them. But if your soil was clean then you could grow a plant from potentially contaminated seeds and that plants seeds would be far less contaminated.
The question boils down to the actual harms you're mitigating and what constitutes the appropriate response.