Filling in a raised bed

we filled our raised beds with a layer of well rotted horse manure but cattle manure is ok if you can get some then we covered that with a layer of spent compost

when i made my Daughters raised bed i filled it with a layer of horse manure then bought a bale of peat to put over the bed  there's not really a cheap option my other Daughter managed to get a load of top soil for her raised beds that someone was getting rid of that cost only for the delivery

I've filled mine with a combination of compost/top soil mixture from my compost bin, horse manure I can collect from a nearby field for free, and bagged multi-purpose compost. If the first two aren't an option for you, if you are constructing paths between the beds, you will likely be digging down a few inches and skimming level, the soil excavated from that can be put in the beds.