I don't have an answer to your question about the authorization form. But I want to mention a couple of related things.
First, you can purchase TIPs through a brokerage firm. I've bought them through both Vanguard and ETrade, and I'm sure most other brokerage firms can purchase them for you as well. No special paperwork required.
Second, have you considered buying them in a retirement account instead? It's more tax-efficient to keep TIPs in a retirement account because in a taxable account you are taxed on the "phantom" income (inflation-adjustment portion) of income each year, even though you don't receive that until the bond matures. If you want to own TIPs in a non-retirement account, you might want to consider purchasing shares in a TIPs fund (like TIP or VIPSX). When you own shares of a fund, at least you receive the inflation adjustment you're being taxed on.