Obeeche can be used, it is a very good comparrison to Sitka. The cost is lighter on the wallet as well.

If you truly can't find a good source near you for Sitka, you can order from Noahs in Canada. A lot of boat builders order from there.

I suggest getting with Darell Sorenson as he surely has a good source for the Sitka. I believe he also sold it for long time, he might still. Can't get a lot closer to you than Sorenson.

If you must, I have used popular for cross frames and stringers. It is heavier, yes. I don't think it reacts like Sitka either, and might be weaker in hydroplane construction. But, it works for the loads we apply to them in the larger cubic inch classes in hydros.

If you only have access to plywood, you can laminte plys together using west system, to reach the desired thickness, typically 3.4" and cut to length. This will be as strong as the sitka, but alightly heavier. Make sure you alternate the grains to achive the load bearing in all directions. And if possible, use a layer or two of Carbon or glass depending on your strength to weight needs.

As a last resort, send me your specs and I can see what I can do to cut your frames and UPS them out.