First and foremost be sure of your terminology and system usages. Is this REALLY a drywell and is it REALLY homeowner repairable? You don't say where this is or what it's application is, but many cave ins are MUCH MORE DANGEROUS than they seem. If this isn't really a drywell, but is a septic tank seepage pit, it could be six feet diameter and fifty feet deep. Are any children likely to be interested in this while you're at the store buying block? PLEASE BE VERY CAREFUL!
If it's really a drywell and it is less than four feet deep, you can probably re-use the existing block that makes up the sidewalls, and will only need a new lid. Make sure it is what you say it is, clean it out look at it and see what the old lid was. Was the old lid concrete or was it made out of redwood planks? If its a concrete lid, contact a LICENSED septic tank manufacturer and have one delivered and installed with a boom truck.If the lid is redwood planks you can rebuild the lid atop the sidewalls, leave a plank off and fill the drywell full with broken bits of concrete, brick, block and large stones and large gravel. The larger the size the better, since the drywell gets water only occasionally, and then the large spaces allow both anerobic and aerobic bacteria to thrive to consume organics in the soil. Completely filling the drywell will prevent later cave in surprises, but will still allow water inflow and settling.
: My old drywell caved in and I'm going to dig up the old one. I want to go to the masonry store to buy the drywell blocks and install it myself. What do I fill the drywell with? Crushed stone? Pebbles? Please give me some advice. I don't want to spend so much money to fix it. I'm hoping it's something that I can fix on my own.