I wanted to get my hands on the tuba part for Bruckner's 8th so I could study the finale. I found I could get the full score for free, but I had to pay for the singular part. I'm pretty sure that the piece is in public domain, so why would I need to pay for specific parts to sheet music?
The piece itself may be free, but the specific engraving used for the part may still be under copyright and licensed to (or owned by) a publishing company. More likely, someone had a public-domain score and uploaded it to IMSLP (or a similar site) but did not have the parts for that score, so the parts weren't uploaded as separate files—there may be public domain parts out there, but no one has scanned & uploaded them.
I have looked at the scores for Bruckner's 8th at imslp.org and the tuba part is indeed available for free download as a PDF. You may have overlooked it because the individual parts for the alto trombone, tenor trombone, bass trombone and tuba are grouped together as a section. You can download the one file (which is 32 pages) and find the pages devoted to the tuba (the final 8 pages of the 32)
Here is the link to download this part. However, you may be prompted to make an optional donation to IMSLP before you are permitted to view or download the PDF file.
Here is the page with all the information on the score and its parts
The tuba pages are labeled "Kontrabaßtuba" which is German for "tuba".