There are various places online that you can get new pro stock pants, but I've only ever bought used ones on eBay or Sideline Swap. I've found it better to identify the model of pants you want, for example "CCM HP45", and search for that. If you only search for pro stock, you often are missing a good number of options. With both Sideline Swap and eBay you can set up alerts to get notified of new items that get listed that match what you are looking for. With a bit of patience, I've had good success getting specific model and color used pants still in good shape for under $100. You can pay $50-$100 more, but I'd rather save the money. And by "save the money" I mean fund other hockey equipment purchases.
That being said, if you have your heart set on an exact color/version that is uncommon it can take a lot of patience and a lot more money depending on what it is (like specific outdoor game or specialty colorways).