the white one? aka this one
edit: image removed for clarification
afaik the same keys for the m fit the spacesaver (basically its a 10keyless).
the keycap+stem and one-piece-molded both fit into the same style mechanism so should be interchangeable. not sure if the one would have a very slight height difference or not so i'd stay with the same sort of design you have already (not sure if you have the k+s or 1piece)
provided the keyboard layout is the same (ie not the L shaped enter key, etcetera) the keys from a full size should fit the spacesaver so if you had 1piece you could change out to all stems.
as far as the black one is concerned (the trackpoint models) they use black keys and may be hard to find keys for (and you'd be stuck with exact replacements)
clickykeyboards deals in all original ibm m parts afaik so isnt a bad choice however they are often sold out of most items (since they have a limited stock). the items they dont have are clearly marked "sold out". another option is ebay such as this one
http://www.ebay.com/itm/Replacement-Keys-and-Keycaps-for-IBM-Model-M-Clicky-Keyboard-Choose-Any-Key-/190795326001?pt=US_Vintage_Computing_Parts_Accessories&hash=item2c6c49a231
if you wanted to change over all the keys from 1piece to k+s (again not sure what type yours is) you sometimes can find whole lots of all keys from an m. or just buy a broken (nonworking) m and take the keys.
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i have two model m's (one from 1987, one from 1991) and while i cant say i'm 100% certain since i dont have a spacesaver as far as i can tell they are just a cut down m and look to have the same size keys (and i know the mechanism is the same). i've done a fair bit of research on the m from repair to modding and have owned and used one a long time so i'm pretty confident those keys will work. but since i havent personally used or owned a spacesaver... its an educated guess.
all of what i said only applies to the white model. not the trackpoint black model. if you have the trackpoint you are screwed and ned to buy those exact keys