I need a list of polyA site positions. For instance, I extracted TSS positions from a gtf file by taking the starting position of all transcript features. Would the end position of the transcript be the polyA site?
Many gtf files contain the coordinates for the 3'UTR, too (check the 3rd column). Its end coordinate might be a suitable proxy.
Poly-A sites aren't typically annotated, since they often don't exist in the genome, but are rather post-transcriptionally added. See, for example wikipedia for an overview.