The total amount of money weighted to issued stock can not exceed the market capitalization because the market capitalization IS the value of issued shares.
What you are referring to would be a case of a highly illiquid stock, one that probably doesn't belong in an index fund. If an index fund cannot buy shares, that means there are no sellers, which means there isn't really a market for the stock.
Markets are made by buying and selling activity and 99.99%+ of the time there are both buyers and sellers.
Think about it from the perspective of an individual investor. There is almost always a price at which someone is willing to sell. If you bought AAPL at $200, there is likely a price you would be willing to sell those shares. It may be $300, $500, or $1000, but there is a price. Obviously, this is an extreme example and highly liquid stocks tend to have much tighter spreads (difference between bid and ask prices).
Also, keep in mind that index funds sell positions as well.