I recommend finding a good condition used copy of Unreal Anthology on amazon or ebay for about $20 USD. Unreal Anthology includes both Unreal I & II along with UT99 and UT2004.
I still haven't played Unreal single player but one day when I decide to I now have the disc after buying a copy of Anthology. Also, UT2004 is really good just not many players anymore 🙁