Any suggestions on where I could find the usual turkey
Any big supermarket. Goose too if you prefer something more traditional.
root veg
They don't really have parsnips (
pasternak) at all and most people in PL have never tasted them. There's something (
pietruszka) that looks rather like them but tastes very different and is used differently - so if you want them, you'll have to bring them.
Gravy
You're a trained chef and asking about gravy??!? There are packets of something called
sos do pieczeni however it's intended for pork and rather bland in line with general tastes in PL. At a pinch, you could use it as a thickener but frankly home-made is just as easy as mucking about with a sachet of something.
Skrobia kukurydziana is in every food shop and similar to cornflour - just don't use as much since it's much starchier. I've noticed by the way that Delia Smith's gravy recipe is cropping up on Polish recipe sites at the moment - British food is becoming quite fashionable now.
sausagemeat
No - they have plenty of sausage, however it is very different. There's
mięso garmażeryjne in all meat shops - you could add breadcrumbs and spices to it, to make a substitute (I've done this) however it isn't anything like the same. Stuffing isn't used much at all in PL but goes down well when you serve it.
Produce
Depends what you mean. Things like sprouts are easily available.
BTW, Yorkshire puddings always go down very well in PL.