In my reading (I highly recommend the book The Other Catholics) and online perusing I've gathered that the boundaries between jurisdictions are often quite soft, with priests and even bishops moving around within the ISM, between formal bodies. I can't imagine this apparent instability is healthy for any of the individual micro-denominations, though perhaps the ones who move around most don't have regular parishes they serve. I simply don't know. There are, apparently, a number of on ramps into this group. One of which is through the Liberal Catholic Church - Young Rite.
The Young Rite organization, which confusingly also utilizes the name Community of Saint George, holds to an 'esoteric' form of Christianity that is less concerned about the historicity or literal reading of the Bible, and more interested in drawing out meaning and applications for the here-and-now. This group is especially eager to confer holy orders for the priesthood, saying "Priesthood is for all, therefore all may become priests." This isn't the same as online ordination as offered by the Universal Life Church, by any means. There is a formation process that seems to take about three years and involves readings, writing summaries of the readings, regular meetings with a mentor (can be via Zoom), and practical training in serving at the altar. Priests in this body are also required to attend an annual Fall synod that involves traveling.
So far as I can tell the expenses for the candidate for ministry are primarily books, vestments and altar materials, and cost of travel for altar training and the annual synod. Considering that ordained ministry in a denomination like the Episcopal Church requires 4 years of undergraduate work and another 3 or so years for a Master of Divinity, along with CPE and internship, the cost is much lower to go this route. Then again, there's no guarantee of a pastoral assignment with the Young Rite or even most of the independent jurisdictions. It appears that the thought is that a priest will either form their own community or serve in some other capacity, such as officiant or chaplain. The doctrinal commitment required seems to be exceptionally light as well.
By the way, the application to start this formation process can be found here.
Another path to priesthood could be found through the Catholic Apostolic Church of Antioch. This jurisdiction maintains Sophia Divinity School (unaccredited) as a primarily distance learning program of formation. Successful completion of all program requirements usually leads to holy orders.
Visit the website of virtually any ISM jurisdiction and you'll likely find pages related to vocations and incardination. The former has to do with becoming a deacon or priest, and the latter is for people who already have holy orders and want to change jurisdictions.
As I've said, none of this is really for me, but if this sounds like your sort of thing, and a way you could express your faith through service to others, then by all means go for it.