Intel provides Quartus editions through their official website. You can download the software by registering for an Intel account. Intel periodically updates its software offerings, and while direct access to version 8.1 might be limited, obtaining the latest or a compatible version is feasible.
Quartus II is a suite of tools developed by Intel (formerly Altera) for designing and simulating FPGA (Field-Programmable Gate Array) and programmable logic devices. The of Quartus II (now deprecated in newer versions) was historically offered as a free, limited-feature version for academic and hobbyist use, supporting older FPGA architectures like the Cyclone, Arria, and Stratix series.
The Quartus II Web Edition is free to use, but it's subject to certain limitations and requirements:
That being said, here's a blog post on the topic: