There was a complete A1 upper on here for $375 last week.
Things to look/ask for:
Is it chrome lined (worth about $50 extra when new)
Barrel twist (1:9 is good, study up on it if you don't know why it's important)
Does it have sights (more an issue for M4 types, but plan on spending another $50 for a carry handle or BUIS if not)