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.