They are "off the shelf" in that you can give them a call and in 6 weeks have one. I suspect they obtain and load the nuclear material for your order, but the rest of the device is stocked as it involves custom semiconductors which have fairly large minimum orders.
These are the class of battery that would replace the CMOS battery in something that can never lose power to its RAM, rather than the much more powerful RTGs that space probes use.
Part of a spectrum from MEMs through pacemaker batteries, Cassini-style RTG, pebble-bed reactors, up to massive GE/Siemens style LWRs. It's a remarkably scalable tech, both up- and down- scale.
CityLabs manufactures specific product lines that aren't special-built.[1] The price is still "contact us" but they are purchaseable and I've heard of a few popping up on marketplaces before.
These are the class of battery that would replace the CMOS battery in something that can never lose power to its RAM, rather than the much more powerful RTGs that space probes use.