Adafruit offers breakout boards based on the family of chips you are considering (ADS1015, ADS1115). They even have some nice code examples for Raspberry Pi.
The most common solution seems to be the MCP3008 chip, which doesn't need any special circuit to work (This is why there aren't many breakout boards for this one, although you may find some).
Update: For a 16-bit enclosed solution you could use an USB DAQ device such as the ones from NI (NI-DAQ) and MCC (USB-1608G Series), althought these are usually more expensive and designed for high sampling rate requirements.