I've been shooting cedars for a long time and have never found 5/16 shafts in 65# spine in uncompressed cedar. Have had some 21/64 cedars that spined that heavy. Yes, they did make 21/64 cedars in the past. I have found some 80-85# shafts in 11/32 though, so 65# in 5/16 should be possible. In truth, i've seldom looked for 5/16 shafts that heavy, except for forgewoods. Just automatically looked toward 11/32 shafts in that weight. However, they may have been and may still be around, but they would be quite rare.
Sweetland forgewoods will definitely spine that heavy. I have some 9/32 tapered to 1/4-inch forgewoods that spine 65-70#. These are extremely rare, however. Likewise, the forgewoods made by Alaska Frontier Supply would do the job, but they, too, are quite rare. As Snag pointed out, they've been out of business for several years.
Might be able to get 5/16 Doug fir to spine 65#@28 inches, and, of course, most hardwoods will spine that heavy in a 5/16 shaft. You'll probably need to go even heavier in a hardwood shaft. They're physically heavier and don't recover from flexing around the riser as fast as cedar (or carbon), so more spine is required. I have some Ramin shafts in 5/16 that spine in the mid-60s. Very nice mid-weight shafts.