Do you just want a trundle bed, or a popup trundle (turns into a double bed?)
For the trundles, you can get them anywhere – I actually think the Ikea ones are really cute. For popup trundles, I found them at a couple suburban furniture stores – I think Slumberland and Hom, maybe? But the cost was high – the only affordable popup trundles I found were just metal frames, not very attractive.
We ended up just putting a new Queen in our guest room because the popup trundles that looked nice were more than we wanted to spend.