A barcode scanner just connects to the PC and acts as a keyboard, so essentially when you scan a barcode it is no different to typing it on the keyboard. Therefore you can easily use for user login.
SambaPOS doesn’t have the feature to generate barcodes, but you can easily create these yourself. The barcode would be the same as the User PIN, therefore if the PIN is 1234, just create a barcode for 1234. There are many free barcode generator websites online, here is one I found from a quick web search: https://www.barcodesinc.com/generator/index.php
As for barcode scanner recommendations, any would suffice, cheap ones will work fine for this type of use. However it is best to go for a laser type rather than CCD, as they are faster to use and more accurate, yet still cheap. Depending on where you live, you can probably find a cheap one off eBay or Amazon.