There's some sites that let you paste an itamatrix itinerary and show you places where you could potentially purchase it (eg https://bookwithmatrix.com/ https:/

There's some sites that let you paste an itamatrix itinerary and show you places where you could potentially purchase it (eg https://bookwithmatrix.com/ https://bookwithmatrix.com/). If that doesn't work and you really want a specific itinerary, I've had good luck with amex travel concierge, where I provided them with copy-paste of ITA matrix and they made the booking.