I need a small size up top and a big bottom... I have a black bikini bottom from Macy's (their swimsuit selection has actually been pretty good for me, or at least the bottoms since you can buy some of them separately from the tops). I have one tankini top of LE from a few years ago, one tankini top from Target- try there! FS/FR with a Redcard, otherwise super easy to return in store since there are so many of them! And bought one recently from Athleta. I like it- it does have a high neck- but I'm petite and feel like the tankini top might go down too long. Plus their stuff is really pricey. I have the tags on it still and should check their return policy and check out their selection online, in the meantime.
Good luck! I hate swimsuit shopping.