I always used to make my own.
Take your dried marrowfat peas and cover with boiling water. Add one teaspoon of sodium bicarbonate per 500g/1lb peas (dry weight, not critical). Allow to soak overnight. Rinse well under running water, place in a pan and cover with water plus an extra 2cm (1 inch) water. Bring to the boil, skim off any scum, and simmer until cooked enough for your liking. This time can vary a lot depending on the age of the peas (select peas that have the longest sell-by date).
Whilst there is still some liquid in the pan do a test for any remaining soda. Add a few drops of vinegar to the liquid and check if it fizzes. If it does, carefully add more vinegar until it doesn't. Don't worry, provided this is done carefully, there will be no taste of vinegar remaining but, even if there is, it is a compatible flavour. Remaining free soda gives an unpleasant taste - I think this may be the reason why some people dislike mushy peas so vehemently.
Keep a check on the liquid level and the degree of "doneness". Add more water if required or boil off any excess near the end of the cooking time.
These freeze very well indeed. What you can do is freeze just before they are completely cooked and the freezing process will do the last bit of "cooking" for you.
I used to make several lbs at a time and freeze in double portion bags. It was possible to buy 5lb marrowfat peas in a cloth bag for next to nothing
If you are making any additions; sugar, salt, vinegar, mint sauce, butter (for pease pudding) do this after thawing.
You can make mushy yellow or green split peas. Basically similar but, being smaller, cook much more quickly.