I'm pretty sure the OP wants to buy individual bonds as most all brokers will let you buy the etf TIP, the mutual fund VIPSX, as well as others from Schwab, PIMCO, et al.
I have a couple hundred thousand invested through Sharebuilder. In that account, I have 140 shares of iShares Barcleys TIPS Bond Funds (TIP). It that what you are looking for?
\I want to open a ROTH IRA and use it to buy TIPS. Which online broker except for vanguard and scottrade that will let me do this?
I opened a sharebuilder account but they're not a TIPS broker.