I've tried several and never had any complaints, so whatever you can find locally will probably work.
dd's first was a Speedo sunsuit from Target. I was glad I bought a little big, as it covered her from her wrist to below her knees. That suit lasted till she outgrew it (like 3yrs!) While the material was faded towards the end, it was completely blocking the sun till the very end.
The next was a Speedo set of a shirt and "boy short" bottom from a yard sale. It was in great shape, but the bottoms have always been too big.
The past 2 outfits have been from the Childrens Place store. I'm pretty far north, so our outdoor swimming season doesn't start till July, so I can usually catch the swim stuff on sale for $10 a piece. I've been buying a sunshirt and then a matching bikini/tankini. That way dd can wear the bikini for indoor swim lessons, and use the sunshirt for outdoor swimming. Doing it this way, I can buy the sunshirt one size up (dd is slim, but has a super tiny tushie and broad shoulders). Overall the sunshirt holds up great, but it gets much less wear than the swimsuit that dd wears weekly for lessons.