There's a suit I want to buy from Peter Jackson online, but it seems like the trousers sizes are fixed with the suit sizes? Example here: https://i.imgur.com/yOXv9vc.png.
I'm assuming first number is jacket size, while the second is the trousers.
Does anyone have any experience with this sort of situation, where my trousers size is different to what they have for my jacket size?
How much does a typical alteration cost at one of those chain alteration shops?
don't go to a chain alteration shop.
usually expensive
find an independent -working-out-their-home alteration place.
The second number is hobbit sizing
This is also my issue with Peter Jackson's online store. I typically buy my suits from them in store or at their DFO stores, because my pant size is different to the one assigned to the jacket on their website. I haven't been able to find a get around for this, but your best bet is to go in to the store and try your luck (sometimes they may have your jacket size, but the pant size is out of stock/unavailable).
My sizing doesn't tend to line up with the set nested suit sizing, so I just buy separates. Also means you can buy 2 or 3 pairs of pants which match the jacket.
Why don't you go into the store?
You get the right jacket and mod the pants skinnier.