first of all, what the heck? 42? pills or boxes?
this medicine is used as a first choice treatment of few stomach infections (girardia, amoeba and so on). usually it is 2 pills a day for three days which is why in DR is not sold individually but by a box of 6 pills, full course of treatment.
it is available over the counter, in any pharmacy, really. but it is not cheap and such quantity of boxes (7) is hard to find in a single pharmacy. maybe in santiago or SD, otherwise you would have to order it beforehand, accept boxes from different labs or prepare for a long wait as the pharmacy tries to find you the same brand in other places.
in any case, in DR you can buy it under various brand names: nixoran, nitazofin, nizonide, nor-tripar, paramix, paraxin, uniplus, zoxanid and gosh, plenty more.
i checked for you the price of uniplus, 363 pesos per box of 6 pills. 42 pills is 7 boxes = 2.541 pesos. if you find it all in one place and pay cash you should get 10% off. otherwise it's full price for ya.
you will have more luck with 42 boxes, in a way. that would be a special order, most likely needing to be paid in advance. but you'd not get it at retail price (over 15k pesos). most places will offer a good deal with up to 20% off (on cash payment).