http://www.walmart.com/ip/South-Shore-Smart-Basics-Bedroo...
Walmart and many stores have sales now, for beds, or child furniture.
Re trundle beds, its not real practical. A friend of mine has one for her kid and they never use it. It would be more useful to get a bed bed, or one that has drawers under it instead of a trundle bed.
Also, if you get a 2nd hand bed or mattress etc., make sure it does not have bedbugs or mites, or anything like that.
Most "kiddie" sized "furniture" are more toddler-ish in proportions. Thus, for a 7 year old, they grow quickly and may outgrown "kiddie" furniture quickly.
My son is 6 for example, and well, he already has outgrown the "kiddie" furniture we have. Thus, we are getting rid of it now.