Twin size baby crib

We started some crib shopping the other week. Meaning we went into the only place that we know of that sells kids furniture. Straight away we found something that interested us- not something that we wanted to buy or would by any means be safe to put a baby in. It was more the idea. The shop had a selection of baby beds that used a twin size mattress. The outside rail (which had cut out shapes for the baby's head to get stuck in) double hinged down. The idea being that it could hinge down easily for putting baby in to sleep and then had a secure lock at the top. The second (mid section) hinge would go down to act as a toddler rail. Eventually the whole side could hinge down and be the twin bed your child would sleep on. I've not seen crib size mattresses for sale here. They have foam that could be cut for a crib but the foam is hot and isn't a firm baby bed. I need to check but a friend said to check the catalogs at baby furniture dealers because they may not have them but they may be able to order them. But in my head it seems a better idea to avoid the whole crib mattress issue and get straight to the deal. A twin size mattress would have a lot longer life span. So I spent some time searching around online and chalked up another win for wasted time- I found nothing. Surely if they are selling them here someone has had this idea before. Have you heard of this idea? Do you have a website I could see such a thing? Our thought is that we could have a carpenter build the thing cheaper than buying one and have a sturdier, safer crib.