What can I use instead of diamond art sealer?

Gloss Mod Podge is the most common alternative: it dries clear. A clear gloss acrylic craft varnish also works.

Hairspray is often mentioned, but it holds less well and can turn yellow. Whatever you use, test it on a corner first and apply a thin coat, as a thick layer dulls the facets. If the picture is framed behind glass, you do not need any of these.