Sounds silly but where can I buy worms?
I looked into this recently for much the same reasons and read up a bit on it. There are different types of worms, the compostery or wormery (also fishing shops) worms are Brandling worms and won't thrive in the soil, although apparently they'll be happy on a compost heap. Put them in the soil and they'll die. You need Earth worms.
I emailed Suttons seeds about this and a nice man called Keith emailed me this back.
"Dear MY Name
Thank you for your email. Brandling Worms are used improve the quality of compost, in a composter or similar. It sounds as though Earth Worms would be better for your purposes. We sell these through our sister company Organic Gardening and I have included a link to their website below. I hope this helps.
https://www.organiccatalogue.com/feeding-soil-care/making-compost/worm-composting/earthworms-average-100.htm"
I read up a bit more, and read somewhere that if you want to introduce earth worms take not the worms up, take a lump of sod with the worms in it up, insert it into your plot soil and let them colonise from there.
Personally I'm trying to get someone to sell me bulk compost to dig into my awful probably entirely sub soil and then I plan on the sod transfer and bribing the kids to worm catch worms on paths when it's raining (distract them from flooding their wellies). Then they can try planting them around the sod with the worms I found in my shop bought compost.
The latter worms shows they don't need sod to thrive and I have no idea how they got across 10 foot of very well drained gravel to climb in there but they must be dead 'ard commando worms. So they should make it in my plot.