Pippa's underwear came with boxed dolls and some outfits, it's quite arbitrary but the general rule is they didn't come with trouser garments (too bulky so she has to go commando). They only come in white, even they look fairly yellowed in the pic – I had to darken the picture so they would show up OK. Indeed, it's hard to find pristine white ones anyway.
Pippa did not wear a brassiere; maybe she burned her bras in a fit of '70s feminist rage or maybe her permanently perky plastic self made them unnecessary.
Pippa's Boutique set (1973) came with a pink bra and pants set. It's very hard to find, but I've got the bra so I'll put a pic here soon.