I will be arriving in Shenzhen Airport on July 28 and going to HKIA on the 29th. Can I purchase shuttle ticket when I arrive in SZ

Do you mean you want to buy shuttle bus ticket from SZ airport to HKIA? If so, of course, you can buy it.
But I think it is not necessary. From SZ city downtown area, you can also buy other cross-border buses' ticket to reach HKIA instead of the shuttle bus from HKIA.