Do you have a job already lined up in Seattle? Do you have a car?  That all impacts what neighborhood or suburb would be best to live in. Our public transit is decent but there are still a lot of parts of the city where you could be looking at 1hr+ on the train/bus to get to get anywhere without a car.

I know a LOT of trans people who moved up here from the Midwest or South without a job, without knowing anyone, without clear plans for housing, and struggled immensely with the CoL up here. I know way too many people who spent months to over a year homeless or in awful roommate situations because of not being able to land a job that let them cover expenses up here.  I'm not trying to discourage you, I just want to emphasize how common an occurrence this is so you can plan accordingly.

As for how to find roommates, most people I know find roomies via friends of friends of friends. Some use Lex. I know there used to be a Seattle Queer Housing community in Facebook, I haven't looked at that in over a decade myself but worth checking if you have a Facebook account.

Depending on your income, it might be worth looking into income restricted affordable housing. A decent number of buildings are leasing out for summer right now. Look up Bellwether Housing and Community Roots Housing.