Just build extensions for the tallest t-posts you can find if you dont need that many 8’ t-posts. PVC pipe or lumber works.
7’ t-posts make about a 5’ tall fence after driving 'em in the ground. Also keep in mind… Deer can jump very high, or very far, but not very high and very far at the same time. More work and more expense but a double fence of a shorter height can be effective too.
Wood posts, cedar ,locust .or other for short term.
Or pipe / posts used for chain link fencing should be available in suitable lengths .
2 inch x 10 ft lengths of pvc electric conduit may work , ( 2ft. In ground )and it may be insulated enough to not require insulators ?