How do you get Volbeat in Ruby?

You can find Volbeat and Illumise on Route 117. They are quite rare so search well, Volbeat are easier to find on Sapphire and Illumise are easier to find on Ruby.
Good luck.

Look in the patches of grass near the day-care center. Sooner or later, one will pop up.

Volbeat is in Route 117, there are few, so keep searching

You can find Volbeat at route 117. P.S they are extremely rare in the ruby version. Volbeat and illumise's rarity switch for ruby and sapphire.

It's on Route 117, but you'll have better luck in Sapphire.

Use illumises(pokemon)sweet scent but it will take some time.
In the patch of grass where illumise is.

I'm not exactly sure, but worked for me is I caught an Illumise, then I put it at the head of the party, and it attracted Volbeat. Try it.

Go to Mauvill and from the pokecenter go right until you find the girl Pokemon breder. Go to the patch of grass under her and walk around a little bit you'll mostly find Rosalia and illumise. Hope I helped I know I spelled some of these words wrong but you can desipher some of them.

You can get a VOLBEAT EASIER IN Pokemon ALPHA SAPHIRE AND you CAN GET ILLUMISE EASIER IN OMEGA RUBY

What worked fastest for me is putting illumise as the lead Pokemon and using sweet scent to attract them. It still took some time but it's better than chance encounter while walking. Do it on the patch of grass nearest the daycare