For sale are three used but clean and perfectly good high back booster seats.
Boosters 1 & 2 are Diono Monterey 5iST (retail ~$250 if you can find one)
(pictures show only one but there are two - one grey and one black)
https://www.walmart.com/ip/Diono-Monterey-5iST-FixSafe-High-Back-Expandable-Booster-Car-Seat-Yellow-Mineral/621284577
Booster 3 is a Peg Perego Viaggio Flex 120 (retail ~$300)
https://www.amazon.com/Peg-Perego-Viaggio-Flex-Licorice/dp/B01LN83EK0/ref=sr_1_1_sspa?adgrpid=186412170477&dib=eyJ2IjoiMSJ9.mdly6QfmMbCUbtsA9pFU7vAPeA9UNTgoenvAcXvTZ-7u48T_MuVquHRUXsjIziWzqdIdZ5TKsvwZztziHCvl2o9JGG-YXbj2l99sI9gSfFCdqqj4A8sSiM6h7He36oV8BTkdEtNoXQ2g12AuaQHaVyi_q6GTKCQg88u_mhceJ0b3D9qhvk_E5UMPriP57ORqZvB518a02sbTAss10yO3baVDoTHwSUO3OPIwUIpdtyKX2BjCnivK8qc43CzY2uySnQfjFGSM8EIVuUZ-QOFqvlNG-MEnBSIgipo5MN2Oa6s.FBFtrSjgxUV0NvojgDfjdfY-5tbmLZjF0ofqaXLtcsk&dib_tag=se&hvadid=792845782487&hvdev=c&hvexpln=0&hvlocphy=9002081&hvnetw=g&hvocijid=1048944769475263695--&hvqmt=e&hvrand=1048944769475263695&hvtargid=kwd-344440342931&hydadcr=15620_13881557_2459161&keywords=peg+perego+viaggio+flex+120&mcid=734655c08658388a803467a575c64c30&qid=1788709117&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1
All three boosters are adjustable and are designed to fit kids 40-120 lbs. Asking $300 but will entertain reasonable offers.
Thanks for looking!