K.R.
Hi. When we transitioned our twin 3 year old girls to their "big girl" beds, we purchased two twin beds at BedMart (Val Vista & Gilbert). Got great deals on the box springs, mattresses and metal frames there, then I ordered some pink padded headboards from WalMart and they have really cute beds! The inexpensive mattresses aren't the most cushy, so I added some egg crate pads over the top. Honestly, I think their beds are the most comfortable in the house.
Merry Christmas!