If you need lighter weights, I'd recommend the 5/16". The ones I've had spine between 40-55#, with the majority in the mid to high forties. You can get them from American Woodcrafters Supply for around $25.00/100, and they make great shafts. You need to remember, though, that the grain is hard to read, and they are dowels, not arrow shafts. Flex each one before you use it to make an arrow with, and you'll be fine.. If you decide to go with the 3/8" dowels, just leave them full length. They'll be heavy, but most of them will shoot just fine.
Here's the post I made on another forum about them.
Well, I was curious, too, so I ordered 50 3/8" birch dowels from American Woodcrafters. They are all straight, 36" long, and look like they would make decent arrows. Some grain runout, and a few knots here and there, but for the most part they appear to be serviceable. Out of the 50, here's how they spined out:
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
Those are WAY heavier than I thought they would be, but I wanted to see for myself.
The 5/16" I have used in the past maxed out at 55#, with the majority going in the low to mid 40's.
Good luck.