The best way to do it is to harvest. You don't need a fermenter to do it, you need any kind of glass or plastic jar or jug, heck people grow starters in beer bottles.
You CAN do this.
All you need is Dry malt extract, water, the dregs of a few bottles of yeast, a funnel, and sanitizer.
1) Bring 2 cups of water and 1/2 cup of dry malt extract to a boil, and cool it off.
2) Sanitize whatever container you want to use, a growler works great.
3) Using a sanitized funnel pour the cool wort into your vessel.
4) Open a beer (I use a spray bottle of sanitizer and spray everything down before I open it.)
5) Pour beer in glass, leaving about 1/2 inch of beer behind. Drink beer.
6a) Pour the dregs of the beer into the vessel of your wort.
b) Drink another beer and repeat step a
c) See step b and repeat as many times as you can handle drinking the beer.
(If I am harvesting yeast and want to save the yeast from bottles over a few days until I have enough to make a starter with, I just flame the bottles after I pour the beer out, spray it with a little starsan, and cap with a fresh sanitized cap. Store in the fridge and repeat the starsan and flaming process when I am going to pour the dregs into a starter vessel.
I've done that with hoegaarden yeast, I drank 2 six packs over the course of a week, and made about 4 mason jars of hoegaarden yeast from the harvest.)
7) Cap container with sanitized tinfoil and let it sit for a couple days.
8) Boil up some more wort, cool it and add it to the vessel a couple days later.
9) If you're not using a big vessel like a growler, after a couple of feedings carefully pour off some of the top liquid, then feed with more dregs until you get a lot of yeast in the bottom of the vessel.
Do it for about a week or so, and then either brew a batch with it (which will reproduce more yeast) then transfer it to sanitized mason jars.
If you make a batch of beer with it, you can then follow the instructions in the yeast washing illustrated sticky and fill a ton of mason jars with all the yeast you need.
I can't stress how damn simple this is to do.
If you can make beer, and open a beer to drink, you can harvest the yeast.