Here's the part you'll need:
I work for a repair shop and we get parts from these guys, so I can without a doubt vouch for the quality, price, and shipping time.
As for replacing it.... Good luck, you'll need it. The only way into the S5 is behind the screen which must be removed to open it up. It's glued down with superSam****adhesive so more likely than not you'll end up breaking the display or separating the display from the glass. If your screen is fine, it's not worth the cost of replacing your entire display just to replace the midframe.
Just in case, here's a display w/ the home button flex: http://www.ebay.com/itm/351067874432