Galaxy S8 will definitely be carrier-locked to Telus/Koodo/Public Mobile.
Best option for unlocking will likely be calling Samsung directly. YMMV, but a colleague of mine got his Galaxy S7E unlock code for $10 directly from Samsung.
Otherwise, ebay Samsung unlock codes generally range $7-14. If you pay more, you're wasting money.
So if you want to use it with Koodo, but are tempted to try to see if it's unlocked by putting in a different SIM - don't. You'll end up locking it to that network and giving yourself an instant headache regarding your shiny new phone.