You're basically describing the same exact problem with a crap trans I had in my Corrado. When you come to a complete stop with the clutch in, nothing in the transmission is turning, but when you roll a little, or let the clutch in and out, you will rotate the gears a bit and get them into a new position to where they will line up enough to slide in. The teeth on the gear and selector hub get ground, and chipped off, which causes hard shifting, grinding/ getting locked out of gears.
Here is fourth out of my trans that didn't grind, or give me any problems, but it had very very bad wear.
Here is what a good gear will look like, notice how sharp the teeth are.
When you are stopped, and it wont go into first, try shifting into 2nd, and then first, I bet it will go right in like butter.
And yes, there is no rebuild kit, because you never know exactly what is wrong and what parts you need till you take the trans apart.
What I would try first, is to get all new shifter bushings, and adjust the linkage correctly. I got the shifter bushings at 4 seasons. It cost about $20 for all of them.
If that fails to fix the problem, then I would drain the fluid, and look very carefully for any metal particles. If you find any, rebuild time.