For short-term and medium-term financing: Local market or Eurocurrency market (for corporations and government ; Eurocurrency: USD deposited outside the USA. ie. petrodollars). Loans are made on a floating-rate basis, for example the LIBOR (they use treasury bills, mini=10 000 USD). There are sometimes differences of quotations with the domestic market, that entails arbitrages.
Interest-rate parity theorem. Valid in the USA and other western countries: if interest rates are higher domestically than in a particular foreign country, the foreign currency will be selling at a discount in the forward market.
For long term: American multinationals are the most akin to foreign direct investment. The American Depository receipt (ADR) is a security issued and traded in the United states to represent shares of a foreign stock.

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