Can I use denatured alcohol instead of isopropyl alcohol when servicing a fork? I want to try to do the lower leg service of my suspension fork myself for the first time. In the RockShox videos on that topic on YouTube, they take apart the fork and clean the parts with isopropyl alcohol.

In the nearby bike or hardware stores, I can't seem to find isopropyl alcohol at all. Will using simple denatured alcohol be bad for any parts of the fork, e.g. the seals?

Or are there any common household alternatives for this purpose?
Yes, denatured is fine to substitute for isopropyl. For regular shop use, the methanol is kinda nasty (debatably) from a toxicity/odor standpoint.

Everclear (which is 95% ethanol) is the other thing from around the house that can sub in.

Isopropyl is more a drug or grocery store item, although industrial suppliers do sell huge jugs of it.