At the risk of going into TMI territory, I just buy human water based lubricant to use, but I always order online because I get weird looks if I walk into a store and buy an industrial sized bottle LOL.
I like this stuff. It's cheap but it works well, and if your horse is unwelcoming to the hose in that area, water based lube is the only way to go.
EDIT: What the heck Amazon
? Links aren't working at all
I'll try again.... Okay, still not working, but if you do a search for Passion Lubes natural in 34 oz, you'll find it on amazon.