The C Fund Common Stock tracks the S&P 500 per this link. You can use SPY to proxy this.

The F Fixed Income Fund tracks the Barclays Capital U.S. Aggregate Bond Index per this link. You can use AGG to proxy this.

The S Fund Small Cap Stock tracks the Dow Jones U.S. Completion Total Stock Market Index per this link. The closest proxy for this is FSEMX according to this link.

The G Fund (Gov't Securities) is tracked by either IEF or TLH most closely.

The I Fund (Int'l Stock Investments) is tracked by the EFA fund.

Other information on TSP funds: http://www.investopedia.com/articles/investing/061113/breaking-down-tsp-investment-funds.asp