fixit.com has iPhone 3Gs displays for $19.99. Those will be used/salvaged displays from recycled devices, but fixit does test and warranty their used parts. They also have detailed videos of how to replace that display, or you could find an independent shop if you don't wish to do it yourself.
They also sell the glass front panel for the 3Gs for $9.99.
The thing that is hard to find and expensive is the digitizer. But if all you need on that 3Gs is a new front glass, that should be no problem at a third party shop.
The original iPhone display is quite expensive and may be hard to find. But working used displays for the 3, 3Gs and up are readily available and not very expensive.
And the newer devices use factory laminated displays where the glass, LCD or OLED panel, and digitizer are all one integrated unit. But the advantage in your case now is that those first couple of generations of iPhone used displays that were separable components.