It’s correct that you can’t buy TIPS through Treasury Direct for an IRA:
You can’t buy TIPS yourself directly from Treasury for your IRA. The rules prohibit it. (You can purchase TIPS directly from the government through a taxable account.) However, you can buy TIPS at auction by placing an order through an online broker such as Fidelity or Schwab. The TIP auctions are in January, April, July and October. The maturities are for 5, 10 and 30 years. You get the best price this way. Online brokers will charge you somewhere between nothing and little for the transaction.
It’s also possible to purchase TIPS in the secondary market, but very difficult to evaluate their pricing. An easier alternative is to hold a TIP mutual fund or ETF in an IRA, but there’s a clanger in that:
The advantage of the TIP funds and TIP ETFs is the ease of buying and selling, low fees on most funds, and the automatic reinvestment of dividend income. Problem is, like all bond funds, the net asset value of the TIP fund fluctuates. It also doesn’t mature at the point you want to cash in your inflation hedged investment. Like all bond funds, the value of the TIP fund will fall if interest rates rise, and vice versa. There is less of a match with inflation. Nevertheless, funds still give you a measure of protection against rising prices depreciating the value of your savings. (ibid.)
Right now it appears that TIP yields are negative, meaning that a purchase locks in the negative yield. If there’s enough inflation, that could still result in a positive net return.
For investors who purchase individual TIPS, the negative yields mean that they are essentially locking in that negative yield regardless of how high (or low) inflation goes. Even if inflation surges, the TIPS principal value is simply rising by the same rate as inflation, but not enough to offset the premium the investor paid (that premium that resulted in a negative yield.)
It also depends on the time horizon. With negative yields, it’s still possible for returns to beat inflation over the short run. Whether one holds individual TIPS or invests using a mutual fund or exchange-traded fund (ETF), total returns over short investing horizons can beat inflation. Although past performance is no guarantee of future results, note that in the 12 months ending November 30, 2021, the Bloomberg U.S. TIPS Index delivered a total return of 6.8%, even though the starting yield was negative.