I went to a kitchen store and got some of these bottles for $0.99. They are freaking amazing. Just a suggestion in case anyone is looking for another option. I especially like how the stream comes out so thin when I go to soap the mop directly.
I just use a old dawn bottle myself. Reanna loves the moerman squeeze deluxe bottle. Anything will work really. Just want to make sure they want be squirting when in your pouch is all.
Yea anything can work, but I need some type of stopper at the top or it will squirt out all over the place. Like @Luke said small dawn bottle works great and has a lid on top. Just your preference at the end of the day, go with what works for ya. Thanks for the share!
We have these also https://windowcleaner.com/shop/ipc-eagle-hydro-bottle-bottle-only
Can’t beat that price, right? 
I have an old Disney ‘Cars’ squeeze water bottle my old biz partner bought me from the Dollar Store back in 2008. I love that thing! It’s really soft plastic so it squeezes real easily. I really like the idea of those condiment bottles, @Stoneface. My only thought is the spill factor. But easy squeezy for sure! I might just have to grab some!