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I've used both a local IP law firm, and Iron Mountain.

The local law firm kinda surprised me that they did it, but it was very convenient for us, and cheap.

As we started selling to bigger companies, we found they were less comfortable with a smaller escrow firm, and eventually one actually demanded that we use Iron Mountain. So we switched to them. They were both a little more thorough, and a lot more expensive, but it still was a pretty minimal cost overall.

(edit: Looks like they've sold that busines -- here's a link I found https://softwareresilience.nccgroup.com/software-escrow/)