Tesco are selling these mini Post-It-style index tabs for £1.20 a pack.
Here's the link. They are the same size as (and look and feel completely identical to) the ones on the Filofax sticky notes insert. So, if nothing else, you've got a cheap way to replenish your stock. But we can do better than that!
When you unwrap the package, you find the tabs are attached to a piece of firm, clear plastic.
This plastic piece is just about the same height as a credit card and slightly narrower.
I rooted out the used iTunes gift card you see above, and used a short piece of tape to attach it to the plastic card the tabs are on.
If you do this right, leaving a slight gap between the two pieces, the clear plastic will sit snugly on top of the gift card. At this point you can put the gift card into one of your credit card pockets, with the attached tabs sitting outside.
I've used a credit card insert, but there's no reason why you couldn't use an empty slot in the cover, if you have one. And if you have a Pocket or Mini Filofax, you could attach the tape to the shorter edge of the card, which should work just as well.
I used clear Scotch tape because that's all I had to hand. My only concern is how well this will hold up in day-to-day use and I may upgrade to a stronger tape at some point.
And if you're looking for other uses for used gift cards, here's a project you could try.