you shouldnt need wotlk to get cata, in gameplay terms, but you might in technical terms.
cata does a lot of reworking to the old-world quests (new class/race combos, new questlines, completely redesigned zones, etc.), so its not like you need to get to 80 to enjoy cata - cata basically changes levels 1-60 and adds 80-85. i'd guess its not really possible to reach 80 (and thus the high-level cata bits) without wotlk.
you can also pick up wotlk for about �8 these days, apparently, so its not a great cost anyway.
no idea if you can still get CEs for wotlk, doubt amazon and play have em, but you might be able to get them from local game shops or whatever i guess...