I get parts from wherever I can get a good price, and I do all the work myself, unless its something incredibly time consuming as thats a commodity im short on these days.
Parts warranty is going to be as per the manufactuters conditions for the part. If its an OEM replacement mechanical part then typically installation needs to be done by a certified tech, otherwise warranty is void. Aftermarket parts may follow similar guidelines, but can vary depending on what the part is and what it does.
Like for ex: a tail lamp housing warranty is most likely valid for consumer install, where a factory ECU or any internal engine components will require certified installation, and you'd need a copy of your RO as proof.
You gotta check the paperwork that comes with the part, or ask if you buy from a dealer (and get it in writing if its an expensive part as sometimes you may get the wrong verbal advice).