I've found <somewhere/some place/a place> where

Solution 1:

Specimens a, c, and d are correct. Specimen d is the most natural. If you omit where (as in specimen b), somewhere ceases to function as a noun (meaning some unspecified place) and instead functions as an adverb (meaning in some place). That changes the meaning. So though you could say that b is correct, b is not a more euphonious version of a.

Here (a), somewhere functions as a noun, modified by a relative (adjective) clause headed by the relative adverb where:

I’ve found somewhere where they apologise to you if you bump them.

I’ve found some unspecified place where they apologise to you if you bump them.

Here (b), somewhere functions as an adverb (kind of like somehow), modifying the verb found and followed by a that-clause (which functions as a noun):

I’ve found somewhere [that] they apologise to you if you bump them.

I’ve found in some place [e.g. in a book or on a website] that they apologise to you if you bump them.

I’ve found [that] they apologise to you if you bump them. (adverb/adverb phrase can be omitted)

Not:

*I’ve found some unspecified place [that] they apologise to you if you bump them. (incorrect; a noun phrase doesn't work here)

Here (c and d), some place and a place function as nouns, each modified by a relative (adjective) clause headed by the relative adverb where. In the context of your sentences, some is an informal version of the indefinite a:

I’ve found some place where they apologise to you if you bump them with your backpack on a crowded tube.

I’ve found a place where they apologise to you if you bump them with your backpack on a crowded tube.