Its a 40 min drive for me to a grocery store w/ NO selection, so I understand where you are coming from. If you have a decent "Middle Eastern" (a term almost offensively broad) section in your grocery store, or better, a local grocery that specializes in ethnic ingredients from that broad geographic region, you have a very decent chance of finding split yellow peas. I don't understand what it is w/ the US and its fixation on green peas. May also find them in the "Mexican" (ditto) section, under the Goya label.If there’s something superior to grocery dried split peas that comes in 5 lb bags or less, please post a specific brand name/ link/ image if you could. I like bumping up their protein, and I’m happy to use more plant protein when it makes sense.
You can also try any of the "cowpeas" or "field peas" for variation, nutritionally very similar. Such as "Black eyed" or "Purple Hull" peas. Navy beans. Pinto beans. Lentils. Avoid the dark skinned beans (kidney, etc). Garbanzos/Chickpeas are close too, but there are enough differences they usually get a separate listing in nutrition tables.
[Edit - I wonder if big Grocers across the pond have an "American" section w/ premade hamburgers, hot dog's that dare not call themselves sausages, and frozen microwave dinners?]