Want to add shelves to old Ikea bookshelf (with hooks for steel side supports)

[itm2]

I have a 15+ year old Ikea Billy bookcase which has open metal sides. The shelves hook over the horizontal brackets at each end of the shelving unit (see photos). I was wondering where I might be able to buy additional shelves for this sort of unit - Ikea no longer sell them - or just brackets that I can screw to ordinary shelves which will allow me to hook them over the brackets in the bookcase.
Could anyone advise?

 

[sircerebus666]

Make your own timber shelf the same shape as one already there and screw it to the uprights

 

[foxhole]

Wrap around and screw to underside of shelf.
https://uk.rs-online.com/mobile/p/p-clips/2988069/