Yes, it's possible. Here's a good list of download links for all iOS versions for the iPhone:
Note that you must download the version that fits your iPhone (3Gs, 4, 4s, ...).
You can find more of these lists by searching the web for something like "ios IPSW download links".
After downloading a so called IPSW file you can simply install it by following these steps:
- Connect your iOS device to your Mac or PC
- Select the device in iTunes
- On a Mac, hold down the “Option” key and then click on
- On a Windows PC, hold down “SHIFT” key and then click on
- Select the IPSW file you just downloaded and click “Choose”
The update should then continue as usual.