Try Bricklink, I've purchased "old" sets there for a decent price. Be sure to check the seller's rating/reviews

If you don't want it new, you can get used sets at places like Ebay.

If you don't need the instructions (Lego lets you download PDFs on their website), you can order individual parts from pages like Bricklink or Brickowl. This is quite involved and can be confusing if you haven't done it before. You need to find a seller (or multiple ones) who have all the parts you need at the correct quantities and colors and who will ship it to your region.

You can buy parts from compatible brands. I've used Webrick before, it works just like Brickowl, but can be cheaper for parts that are rare in the Lego world.

You can also look for sets from compatible brands, like this one. Sometimes they are copies of Lego sets (which I find questionable), sometimes they are unique designs. The quality of those can be hit and miss.

Alternatively, do retired sets come back into circulation again?

Usually not, but sometimes it happens, like with the 2017 Saturn V (21309) and the 2021 Saturn V (92176).

This is the site that I know for old sets:

https://www.brickeconomy.com/

I believe there is a site where you can buy individual pieces to match set instructions but I can't find it right now. Unique pieces to the sets would be difficult but you could get close.

I don't think old retired sets come back but they do release reimaginations of sets so something like this could come back.

Alternatively, you could just get a bunch of loose lego and build your own towns every year. Get a lego train set to add in too.

Bricklink is a Lego owned eBay specifically for reselling sets. Might be on the expensive side since it’s a bunch of flippers, but sometimes you can find good deals