The Infusion class feature refers to specific infusion descriptions to declare what mundane objects can be infused, but the Replicate Magic Item infusion option doesn't appear to specify - all the other infusions have it immediately under the title. With that in mind, we're going to have to fall back on judgement calls - there doesn't appear to be a clear answer in the text.
It's reasonable to assume that the Replicate Magic Item feature requires a mundane version of whatever magic item you're making. With many magic items, this is perfectly clear - they're a weapon, a cloak or a jug.
The two items you asked about are referred to in their descriptions as a "prosthetic limb" and "prosthetic appendage."
I would rule that a mundane prosthetic is required - for example, a Prosthetic Limb infusion could turn the stereotypical pirate's peg leg and hook hand into a flexible leg and hand that work just like the user's original appendages.
I definitely wouldn't default to using a flesh and blood limb, as the items are described as prosthetics rather than reattached mundane limbs - it might be possible for a player to find a situation where this would make sense, but that feels like an exception to me.
Getting deeper into judgement calls, I think I'd say that the important point is that a mundane prosthetic can be attached to the appropriate part of the body, and this is what's required for the base item. I would also allow these as a relatively simple application of an appropriate artisan's tool proficiency, and it would be appropriate to make that easier if appropriate materials already exist - it's easier to make a prosthetic hand from a gauntlet, as those are already designed to attach firmly to an arm.
I would not allow an artificer to infuse some scrap metal and gears, as these aren't a mundane version of a prosthetic limb; but I would allow an artificer with scrap metal and gears to use their proficiency with Tinkerer's Tools to fashion a prosthetic limb, and then infuse that. The main differences here are that building the limb would take time and may call for an ability check, but also that it will remain in place even if the infusion ends for some reason; it's still a mundane prosthetic limb, whose function is to stay attached to the body and help the user move - it just won't be magically articulated any more.
Since mundane prosthetics are not (to my knowledge) addressed directly in the text of any rulebooks, working out appropriate time and costs for building one is also down to GM judgement, and may depend on how common you envision them being in the setting.