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When the creature is first created we get a strong impression that it is built up from body parts, quoting from chapter 4:

I collected bones from charnel-houses and disturbed, with profane fingers, the tremendous secrets of the human frame. [...] The dissecting room and the slaughter-house furnished many of my materials; and often did my human nature turn with loathing from my occupation...

The creature then demands a female companion, and in chapter 19, Victor sets out to create one on a tiny, remote island (emphasis mine):

With this resolution I traversed the northern highlands and fixed on one of the remotest of the Orkneys as the scene of my labours. It was a place fitted for such a work, being hardly more than a rock whose high sides were continually beaten upon by the waves. The soil was barren, scarcely affording pasture for a few miserable cows, and oatmeal for its inhabitants, which consisted of five persons, whose gaunt and scraggy limbs gave tokens of their miserable fare. Vegetables and bread, when they indulged in such luxuries, and even fresh water, was to be procured from the mainland, which was about five miles distant.

Clearly, such a barren rock would not have an abundance of dead bodies that Victor could use as a source for the new creature, and bringing them in by boat to such a small community would like raise too many questions. It is however clear that he does build something, since he destroys it after being reminded of his first creation:

I thought with a sensation of madness on my promise of creating another like to him, and trembling with passion, tore to pieces the thing on which I was engaged. The wretch saw me destroy the creature on whose future existence he depended for happiness...

Is this ever explained in some other version of the book, or in external sources?

The beginning of chapter 19 says that Frankenstein spent six months in London assembling the ‘materials’ for the new creature, before setting out for the Scottish island.

London was our present point of rest; we determined to remain several months in this wonderful and celebrated city. […]

I now also began to collect the materials necessary for my new creation […]

We had arrived in England at the beginning of October, and it was now February. We accordingly determined to commence our journey towards the north at the expiration of another month. In this expedition we did not intend to follow the great road to Edinburgh, but to visit Windsor, Oxford, Matlock, and the Cumberland lakes, resolving to arrive at the completion of this tour about the end of July. I packed up my chemical instruments and the materials I had collected, resolving to finish my labours in some obscure nook in the northern highlands of Scotland.

It is not clear if he took these ‘materials’ with him on his tour of Britain, or whether he left them in London and later sent for them to be delivered once he had found a suitable island. Presumably they were packed into cases or crates and the boatmen would not have been aware of their contents.