In our campaign, one of the items handed out allows the attuned person to prepare 1 extra spell per day.  I cannot find this modifier under homebrew items or feats, my go-to's for custom abilities.

Does anyone have a recommendation on how to accomplish this aside from custom classes?

There is currently no known way to increase a PC’s number of prepared spells with either an item or a feat. It’s not technically possible at all, but there is a way to fake it (sortakinda) with a homebrewed racial trait or subclass feature (ish…). Follow the instructions in the Subclasses & Races FAQ #14 & 15, making sure to select “Yes” in the “Always Prepared” field of the Spells subform.

There are lots of ways to add more prepared spells in 5e.  Taking feats, being a race, multi-classing, etc.  Not to mention things like wands, epic boons, and simply DM fiat.

But there is no way in D&D Beyond to modify a character to simply add more prepared spells.

D&D is not D&D Beyond.  Do not mistake software for the game.

Quote from Mog_Dracov >>

There are lots of ways to add more prepared spells in 5e.  Taking feats, being a race, multi-classing, etc.  Not to mention things like wands, epic boons, and simply DM fiat.

But there is no way in D&D Beyond to modify a character to simply add more prepared spells.

D&D is not D&D Beyond.  Do not mistake software for the game.

 
Truth.

where is the faq? 

My apologies for not having linked it: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). All the FAQs are in the very first post, just expand the corresponding spoilers.