There are three things I have to buy new for myself and the kids because they're hard to come by (or hard to come by clean) secondhand. Sometimes I have to buy new shoes. More often I have to buy new pajamas. I almost always buy swimming suits new (although I did get myself a great used Speedo for bottom dollar on eBay once).
It's February, which is the perfect time to find them super cheap for you and the girls. The bad news is that the Birqini is a hot commodity and never goes on sale. I showed this picture to my daughter and asked her what she thought. She said, "It doesn't have socks." Great kid.
Motherwear is having an end of season clearance including nursing swimming suits for crazy cheap if you know how much they usually cost there. Having attempted to nurse a very annoyed baby under a t-shirt while wearing the aforementioned Speedo at a waterpark, I can vouch for the fact that one of these would be real handy to have if you're looking at a lot more summers of nursing. Coverage looks pretty decent, although the jerks charge separately for the skirts. Sizes and colors are running out, so order while you can.
For the girls, there's a great overstock sale at Land's End. I'm splurging on the 50 SPF Swim-mini skirt bottom and Short Sleeve Rash Guard top for my Scandinavian princesses, and hope to discourage pervs in the process. I hate dropping this much cash on anything, but I really want them to have decent swimwear that falls somewhere between eye candy and Amish. This is as well-priced as you can hope to find it if you have this problem every summer too.