I used to buy empty cartridges from a store in Flint. They were in a big candy-type jar and were unlabeled. They worked OK. I got on-line recently and and did some research and found out that cartridges are available with a variety of oil intake hole diameters. The ones I was using were probably about 1mm. About 2 weeks ago, I bought some cartridges with 2mm intake holes. They are fantastic. The first time I took a hit I was overwhelmed. The oil I use is extracted with butane, but I don't do a fast extraction and don't winterize, so the oil is about the viscosity of tar at room temperature - probably about what you'd get with an alcohol extraction.
I bought 10 for $35ish shipped. They are reusable. https://www.ebay.com/itm/Wood-1ML-5ML-Glass-Ceramic-Coil-Wickless-Vape-Cartridge-510-WHOLESALE/123702988231?ssPageName=STRK%3AMEBIDX%3AIT&var=424297918694&_trksid=p2057872.m2749.l2649
I highly recommend the ooze battery units. You'll want one with adjustable voltage. Some have a manual dial to adjust voltage, and some have adjustable voltage by pushing the button a few times. My personal preference is the manual. Also, the ooze units have a preheat function, which is very helpful for thicker (pure/uncut) oils. Crank up the volts and vape!
I've spent a lot of time and money doing trial and error with vape pens since about 2010. I started with Omicron products, which were kinda good at the time but it's almost like comparing the computer I bought in 1993 with an Intel Pentium 60 processor for about $3,000 to my current machine that is 10 times better at about 20% of the cost.
Resto might add to this discussion. Maybe his extraction/winterizing works well with the common/smaller intake hole carts. In any event, you have a lot of options and are asking good questions in a forum where you'll get good answers.
The carts I'm using have the 2mm holes. Alcohol extracted, 12 hour full extraction with heat, then double winterization filtered cannabis oil. Thick and dark. Do a double cycle pre heat and it vaporizes great and it's a smooth hit. The only small problem is you have to hit it hard the first time because it's slightly plugging the breath hole until you get air through it. And don't tip it up, horizontal, or it gets on your teeth. Good for a laugh with a newby. Go look in the mirror. lol
Man have we come a long way vaporizing cannabis oil. I can tell you some stories.. It's so easy now.
A lot depends on the type of pen you are using. As you can see, I have used many different types. I discuss the different methods I use in more detail in my blog. Trial and error, and personal preference also plays a big part in techniques.
You don't have to thin it. I winterize filter it and it works great in the right pen. I use an ooze pen with the high setting of 900. Highlander and I posted in a thread about how we use it and which cartridges to use.