Ramin is no longer being imported into the U.S. because of ecological concerns. Some may still be in the pipeline, but I understand when that's gone, it's gone.
I have used birch to make arrows, and it works well. The heaviest 5/16" birch dowel I have seen was 55#. Most, as Hormaon said, run in the 40's.
The 3/8" birch runs pretty stiff. I recently spined 50 of them, and this is what I came up with:
68-1 70-1 73-2 75-3 76-2 79-1 81-1 84-1 87-7 90-2 93-4 96-7 98-1 100-5 104-8 108-3 130-1
By the way, just for grins I made up a shaft from a 108# dowel. At 32" with a 160 grain point and no finish or feathers, it weighed 775 grains!