Either get a junk yard trans, or give APD automatic transmission parts in atlanta, ga a call, 800-241-2910.
The next step is get a B&M shift improver kit which will supply a separator plate to upgrade your trans for chrisper shifting. Double check the plate is supplied on the 67-69 C-4 kit, since your valve body is different check before you buy. The best thing to do is get a valve body from a 67-69 to get all the shift improvements.William