L.F.
We bought our daughter's bed frame from a place called Beds Beds Beds! in Downers Grove. It is/was a chain, but they may be out of business now. But most of these bed stores have loft systems. The bed frame we bought is the bottom part of a loft system because it is super low to the ground. Even if she falls out of the bed, she doesn't have far to go. We only paid $99 for it. You can always buy the top part of a loft system later if you need the kids to bunk too.
The mattress we got her is a memory foam mattress. I LOVE this mattress. It is so comfortable. We ordered it online through Costco and it was delivered with the mail in a box that looked way too small to hold a twin sized mattress. Another nice thing about a memory foam mattress is the fact that it is not bouncy. The kids don't try to jump on the bed because it is no fun.
Good luck!