I also have this question.

My goal is to do the following:
#1 Integrate Stripe as the payment gateway
#2 Stripe will be utlized in the POS as well as for online payments for customers AND for employees to accept payments via phone
 

Here is what I have found thus far that may help:
Odoo makes a device named POSBox. It's sold individually and as a kit. This seems to be the easiest way to connect your periphials together:  https://www.odoo.com/shop/product/point-of-sale-posbox-kit-cash-register-318

Regarding the credit card reader (For a Computer/Device with a USB Input):
A credit card reader is essentially just an input device like a keyboard. If you're planning to connect this to a computer, any USB Credit Card Reader should work. I found a decent one with good reviews on Amazon:
http://www.amazon.com/gp/product/B00EUMXPC4/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B00EUMXPC4&linkCode=as2&tag=mpco-20&linkId=UDOTPDX6N52JPYRL

Regarding the credit card reader (For an iPad):
I ASSUME that any credit card reader (like the one from Square or Intuit) SHOULD work. Unless they have locked the device down to only work with their apps. I wasn't successful in finding a generic credit card reader for an iPad online. We will just have to test and see if this works. Maybe someone else can offer more advice?