Boulter, ranked 49th in the world, upset US third seed Emma Navarro 6-3, 6-1 in 74 minutes while sixth-seed Kostyuk ousted US top seed Jessica Pegula 7-6(4), 6-1.
Each of Sunday's finalists seeks a second career WTA title. Kostyuk, ranked 34th, won last year at Austin while Boulter took her lone crown at Nottingham last year.
Boulter, whose week in Southern California has included an upset of second-seeded Beatriz Haddad Maia, was in control from the outset against 26th-ranked Navarro, who won her first WTA title in January at Hobart.
Boulter broke twice to pocket the first set. She was up 6-3, 1-0 when a sudden shower briefly halted play.
The short stoppage didn't help Navarro, who surrendered a break when they resumed as Boulter took a 3-0 lead in the set.
Boulter's first hiccup came in the fifth game, but she saved two break points with thumping serves and held for 4-1.
She polished off the victory with a love game, punctuated by a booming overhead on match point.
Kostyuk had lost her first two career matches against fifth-ranked Pegula, most recently in last year's Australian Open.
Pegula was chasing a fifth WTA title in her first semi-final of the year. It was her first event since the Australian Open after withdrawing from Doha and Dubai with a neck injury.
