If you're looking for a video that's entirely "armpits," I think the MTP one mentioned here is about the only one I know of too. And I agree, it's good.
If you'd like to know of videos that have enough terrific armpit tickling in them that it makes the whole video worth buying, let me make a few suggestions:
Start with another MTP classic, "Farra's First Time." Mentioned in several other posts, Farra is among the most [upper body] ticklish models I've seen filmed. The scene in which she's tied laying down on her back and tickled to the point of snapping her restraints (a tough feat in this one) is a classic.
TC Videos (formerly Femfeet) also has their share of "must-see" scenes. Check out both "Variety of Giggles," and "Dungeon of Happiness." In VOG, the final scene has Sara (yes, THAT Sara) tied on her back with her pits being tortured more than in any other video she's done. In DOH, Rachel gets it while standing and bound to a cross. This is perhaps Kristin Lea's most ruthless scene ever as a tickler, and she's relentless on Rachel's armpits. Quite a long opening scene too!
Paradise Vision's done a good job too. In "Spy Interrogation," Lily has a scene where she too is tied standing and the armpit and rib tickling is to die for (I think she almost did!) In "Family Bonding" Gabrielle has two scenes where she ruthlessly tickles Renee's armpits-- once while on the floor, sitting on Renee's arms, and then again with Renee tied to the bed. Long and relentless scene's, both. Also, try the first scene (if I remember right) from "Ticklish Virgin," where Summer (the winner of their infamous "The Contest"0 is tied to a tabletop and her armpits and upperbody get as strong a tickling as I've seen on video (yes, I'd rate this as the most intense scene of all of these). It's intense, relentless and Summer's tortured reaction is not to be missed.
Finally, Yaqi has done several good scenes too, but the one that stood out for me was from "Erin Loves to be Tickled." She, I believe, is the one featured in his documentary [which I've yet to see], but in this one, Yaqi is relentless on her armpits, and her reactions are what you're looking for, I think.
There are others I've liked too, including some earlier one's from TC with Priscila as the tickler, but I've tried to limit my comments here to videos I believe are still available for sale on these respective websites.
Any others I've missed, people?