Do you have a TV around with Coax, composite, or s-video in? If so all you would need is a cable that takes the C-64's AV DIN connector feed and breaks it up into composite (yellow RCA plug) or chroma/luma (for S-video, preferred) and audio. Failing that, you can hook it up through the RF connector (the channel 3/4 thingie) with a small RCA-to-coax adapter using any RCA-style cable. It will be blurry but might well be what you actually used back in the day and might be the cheapest option.
You can, of course, buy an HDMI up converter ($20-$350 depending on your level of retro-crazy) and use the same DIN-to Composite/S-video cable ($10) I mentioned in the beginning if you only have HDMI as an option. all this stuff is available on Amazon.