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 a raspberry pi compatible USB DAQ device such as the ones from MCC (USB-1608G Series) or the LabJack U6, althought these are usually more expensive and offer a limited ammount of input channels.
Also here is a shield for LTC2499 chip (24 bit, 16 channel ADC), although. Note that this one works with 5V so you would need to use a logic level converter or an arduino board (or another uC) to handle it.