You can use the Pick N Pluck foam that comes standard with Pelican cases, where you have to customize and remove the foam yourself. It's pretty straightforward, but can be easy to fuck up. If you fuck up too much, you'll have to figure out a quick fix, or order replacement foam. This is a good, cheap option as the foam comes with the Pelican case unless you purchased a "No Foam" case. Once you remove the material, it's not going back in, so measure twice, remove foam once and you are good to go. (I have extra foam for a lot of my cases so I can set them up in more than one way.)
Pelican Pick N Pluck:
Another option would be to purchase Pelican's padded dividers if they are a valid option for the case you are looking at. Some cases will not work with padded dividers due to size restraints. These are a good way to go if you want to spend the money, they can be setup to securely hold a ton of stuff and can be rearranged at will.
Pelican Padded Dividers:
A final option would be to contact a custom foam manufacturer and have them design a drop in foam insert for your specific requirements. This is an awesome option as you get to select the density of foam, number of layers if applicable and so on. You will need exact measurements of the case, as well as any items you are trying to have secured in custom foam. Some companies prefer pictures and measurements, but you'd have to contact them for more details. This is a great option if you have exhausted the other two options, and have money to burn, or really want the best possible protection for your gear. Setup charges for custom foam are not cheap, be very aware of that! It could only cost you $20 for custom foam, but potentially $600 in additional charges for the setup involved. I honestly wouldn't go this route unless I had terrible experience with both the Pick N Pluck foam AND
Pelican's padded dividers. Thankfully, the standard foam has worked well for me in the past. And if you've seen most camera bags, padded dividers are the norm for a reason. They work well, and are extremely versatile if you are changing up your setup frequently.