h-node.org has lists of scanners and printers that work with Trisquel, libre.thinkpenguin.com sells printers and all-in-ones that do.
I don't know much about the dots, they probably need special equipment to be detected, so they aren't known for many devices. However, the ones I knew about were only colour laser printers, so e.g. monochrome ones are ok. I have a CanoScan LiDE 110, it works in Parabola except for the buttons (which are supported by SANE, I just don't know any programs that can use them). My printer is Lexmark E120 (mono laser), not expecting it to be still widely available since I bought it in 2007.
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