The best compost you will find is that which you make yourself in your own compost pile. Landscape suppliers frequently sell compost varying greatly in quality. You should inspect it carefully before buying a truck load. Gardeners mix is usually 50% or more sand or dirt, specifically NOT recommended. I have to ask why do you want to lay down a layer of compost? I am going to quote myself from a recent post here. "Although I am a huge proponent for using compost, in this case I wouldnÂt recommend it. The reason is cost and labor requirements. Top-dressing a lawn takes one cubic yard per 1000 square feet of lawn. How many yards would you need? It has to be spread by shovel and wheel barrel, dumping and flinging it around the yard and then raking or sweeping it for thin, even coverage. An alternative to spreading compost would be to mulch mow lots of fallen leaves into your lawn in the fall. This would add quite a bit of organic matter (OM) to the top of the soil where nature places it. Switching to organic fertilizers especially grains would add OM and be good. There are many to choose from. For me, the workhorse of organic fertilizer is soybean meal. It is rich in nitrogen and micro nutrients, is cost effective and usually available at farm animal feed stores. Many like to diversify their fertilizers by switching between soybean meal and corn meal or cracked corn, Alfalfa pellets (rabbit food) is also used. Commercial organic fertilizers are available from Scotts, Milorganite and others. Both are generally available at Lowes, Home Depot, or Mennards. Used Coffee grounds are very good and available free for the asking from Starbucks and other coffee stores. Look into using compost tea and liquid molasses to jumpstart and feed the micro-organisms living in your soil. Cultural practices are very important. Mow high and water deeply but infrequently. Congratulations on your decision to switch to organic lawn care. Have you started your own compost pile yet?
Bill Hill

Here is a link that might be useful: Organic lawncare FAQ