Kuta Software has worksheets divided up by topic. The answers are on the last pages of each worksheet set, but there are no step-by-step solutions.
IXL has practice problems. For Pre-Algebra, check out their sections on 5th and 6th grade math.
Math.com has practice problems for all levels of math.
PreAlgebra page: http://math.com/practice/PreAlgebra.html
There are also some relevant problems under Hot Topics and in their Everyday Math section.