Not neat essential oils? I’m guessing you are talking about diluted essential oils in a carrier oil?
I warm oil for massage by decanting some into a glue bottle, with a long spout and I drop it into my hot stones heater to warm up/put it into an insulated container filled with hot water/a wax pot containing water. I drizzle hot oil onto the client and it feels amazing.
You only want to heat a small quantity, not a whole bottle.
There are lots of methods including wax melts heaters or heated essential oil diffuser. There are lots of improvs you could try to warm products. For a single portion of products you could try decanting product into hot egg cups/mustard servers/tiny sauce bowls or something similar - heat the bowls by having a very hot insulated mug, or similar and filling with scalding hot water. Pop your bowls in for a few minutes, lift out with tongs and then pop a little product into your hot bowl. Keep warm by using a child’s hot water bottle wrapped in a towel as your “serving” tray.
While you are warming products you could also warm one or two facial stones and experiment with adding these to your service. Lava shells do a facial routine - massaging with heat really gives a lifted result. Or why not wash feet with a heated wet mitt? Or do a hot towel foot wrap.
Remember oils are flammable, especially essential oils. Avoid dry heat and naked flames. And be careful not to dry heat fabrics soaked in oil. Also check temperature carefully with a thermometer as 60° is enough to cause a burn.
Enjoy