Adafruit offers breakout boards based on the family of chips you are considering ([ADS1015](https://www.adafruit.com/product/1083), [ADS1115](https://www.adafruit.com/product/1085)). 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](https://rheingoldheavy.com/product/breakout-board-mcp3008/)).

**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](https://www.mccdaq.com/usb-data-acquisition/USB-1608G-Series.aspx)) or the [LabJack U6](https://labjack.com/products/u6), althought these are usually more expensive and offer a limited ammount of input channels.

Also [here](http://www.iascaled.com/store/ARD-LTC2499) is a shield for LTC2499 chip (24 bit, 16 channel ADC), although this one works with 5V so you would need to use a [logic level converter](https://www.sparkfun.com/products/12009) or an arduino board (or another uC) to handle it.