Go to a pharmacy and look at the oral or irrigating syringes. These aren't controlled, behind the counter, or eyebrow-raising products, so getting them is easy. There are some oral syringes that work out of the box, but most other ones will need a little knife and sandpaper work to be comfortable/safe used anally. You should be able to get them for well under a dollar, and online might be another option.
I like to pull out the plunger and pour the lube in so the syringe doesn't contact the lube. Not a big deal for the first use, but if you reuse I don't like having the "dirty" syringe in the somewhat-clean lube.
I'm with you on inside lubing for anal play - it's much nicer if you get some lube inside, and squirting it in is much easier than trying to squish it in with fingers. Prostate toys and plugs move around more when your inside is lubed, and dildos and fingers are more comfortable. I imagine this extends to penises too, but can't say - inside lubing is definitely not just for men, though.