You can buy TIPS from auctions through TreasuryDirect or a brokerage account. Fees and minimum purchase amount vary.
If you are going to buy TIPS in a taxable account, you can set up a free online account directly with the U.S. Treasury. That service is called TreasuryDirect. You link a bank account with TreasuryDirect. All your purchases will be debited from the linked bank account. There is no fee for buying TIPS from auctions through TreasuryDirect. The minimum purchase is a low $100. If you don't have a TreasuryDirect account, you can view its guided tour or open a TreasuryDirect account.
However you cannot establish an IRA with TreasuryDirect because the U.S. Treasury is not in the business of acting as an IRA custodian. The TreasuryDirect account has to be a taxable account. If you'd like to buy TIPS from auctions in an IRA, you have to use a brokerage account. Fidelity, Schwab, and E*Trade are good choices because they don't charge any fee for buying TIPS from auctions as long as the order is placed online. You can also buy from Vanguard Brokerage Service, but you may have to pay a $10 fee unless you have over $100,000 with Vanguard.
The following table shows the fees and minimum purchase amount for buying TIPS from auctions through TreasuryDirect and several discount brokerage firms. All data are current as of April 2009. The link for each brokerage firm points to its fee schedule.
 TreasuryDirect is a 100% online system. There's also a Legacy Treasury Direct service which takes phone orders but you can only buy 5-year and 10-year TIPS through Legacy Treasury Direct, not 20-year TIPS.
 Free for Voyager, Voyager Select, and Flagship clients.
 $15 for Voyager and Voyager Select clients. Free for Flagship clients.
 WellsTrade does not take online orders for TIPS auctions at this time.