Try a salvation army or a good will. I'd try salvation army first tho. But I'd splurge on a new mattress from Walmart just so you know its nice.
You could get a nice new looking crib for around $30 at salvation army. And a new mattress for $30 at Walmart.
I believe it's 6 months that you have to transition by. Either way I know that's what's "best". Where are you from? (Easier to answer the second question that way)