Currently there are no official installers or releases for nix/nixos for armv7l. You gonna have to build most of the stuff yourself and this is only really feasible on an existing armv7l system. Cross compiling is buggy and emulation too slow. I released a nix binary tarball and a guide here: GitHub - DavHau/nix-on-armv7l: Nix without NixOS on arm32v7 / armv7l / Raspberry Pi OS / Raspbian .
In general armv7l is not a nice experience. You gonna spend a lot of hours waiting for builds to finish since the currently available binary cache is quite limited.
Better to go for aarch64 which is officially supported. If you have a raspberry with raspbian you can easily switch it to aarch64. Scroll down here for some more info How to install nix package manager on Raspbian? - #11 by janat08 . Or just flash a full aarch64 NixOS onto your hardware right away.
The people on the #nixos-aarch64 irc channel are also very helpful (also with armv7l)