I'm not sure what kind of elastic you mean. However, I can recommend clear invisible elastic -- very soft and is very suitable for swimwear. Haven't purchased it in years, but I used to be able to find it on the notions wall at JoAnn's. Found a photo of it on eBay:
http://cgi.ebay.com/NEW-1-2-inch-CLE...-/370488691941
I think the kind I used to buy was 1/4-inch wide, though. A simple zigzag through the middle of the elastic works well. With swimwear you would sew it on that way, then turn the fabric edge and enclose the elastic inside the seam.