I started using these 3-tab dividers that I got in packs of 10 for (US) $8.99 each pack from Sleeve City. I started with 3 packs, but most recently got 5 more.
My method is usually if I have 3 or more records from any one artist, they get a divider. There's a couple where I may have only 1 LP from them, but I really do plan on adding more.
Just the top of one rack, I have two and they each have a lower level.
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