Do you guys have anything special you use to hold liquids in that is easy to cap and pour from. I'm talking liquids such as thinners, mineral spirits, pre-mixed paints, etc. Thanks.

if your not going to pur it out then camera film cases are good, and air tight. If you are going to pour it then you could use herb and salt containers. The little refill types that you get at them super markets.
if your looking for a large bottle then try the bottles of water and sports drinks that have the sports nozzel ( the one that you pull out to open and push down to close)

If any of your friends have new babies ... contact them. All those little baby feeds and stuff come in an assortment of different size bottles.
Another option is most hobby shops have little bottles that fit cheap airbrushes. The lid is screwed off and a lid with adaptor for the airbrush is screwed on. I have a few of these but only use them for storage. Can cost, but perfect if you only need a few!
I use humbrol thinner all the time and any washes i mix up I keep them in old empty bottles.

Glass bottles seem to work best for any solvent based product. Film canisters work well too. I tend to save paint jars to reuse on occasion.

General sir, You need to find yourself a diabetic friend. I'm diabetic and the containers that my test strips come in work great for storing liquids. They have to be airtight to keep the strips fresh. Also I have access to hypo needles which are invaluble in modeling. Best of all my insurance pays for them.