I've read from the reviews that the Lepin sets tend to have a couple of parts missing / malformed. Where's the best place to get replacements? Bricklink looks good for the set specific ones, but if I need three flat 10x2 grey panels for example, is eBay the best source? I can raid a few boxes, but have very little grey stuff and the super star destroyer is pretty much all grey!
Brick link is the best place to go looking for replacement Lego bricks etc.. If bricklink hasn't got some seller listing the item you need then its extremely rare - but even then the extremely rare parts are on bricklink. The super star destroyer has a bucket load of very common parts, if I recall the only really rare parts are the L shaped Technic type stud with hole brackets that hold the engines in place, they are in yellow, and are expensive, but the black versions are common however I very much doubt you'll need those.
Mostly like you'll need a panel or something, so bricklink will be best place to go. The SSD is an amazing build, I have the Lego version which I recently got out of storage and built, its very long, so make room somewhere to store and display