Buying SLC outside of enterprise channels is now very difficult and buying just one, low-capacity SLC enterprise drive as private individual is not going to be cost-effective, nor worth your time. If you're looking to use it solely as an OS swapfile location, look out for an old Intel Optane M.2 accelerator on ebay or something; They came in 16GB and 32GB capacities and have huge IOPS and endurance.
If you want higher-capacity performance and endurance, you should still be able to find MLC NAND - the 970 Pro is what I normally use for write-caches in rackmount storage that gets hammered for terabytes a day, petabyte(s) a year. The downside is that they're getting harder to buy and 512GB is the smallest size they make.