Where in Illinois are you?
I'm also in Illinois. I have a few 15 clones and I just picked up my first 15-91 last weekend. It's currently undergoing a "spa treatment". I found mine in an antique store in Morris in a cabinet. It was the cabinet I was interested in and when I flipped the lid ... lo and behold sat an almost perfect 15-91. There happened to be an electric outlet right there so I plugged it in to check it out. The motor ran and the balance wheel turned, but the needle bar did not move. I tried to tighten the balance wheel stop and it wouldn't work. I talked to the guy and told him, and demonstrated, that the machine did not work and got it for 1/2 price. It was listed at $110 (with the cabinet).
Mine did need some work, and I'm still working on it. The problem with the balance wheel turned out to be a missing washer .. which I found in one of the drawers. I had it sewing but the tension was not to my liking. Knowing I was going to strip the tensioner down for a good cleaning (now in process), I didn't try to adjust it at all as it would have been moot.
I'm doing all the work myself. I'm not a machine technician although I am mechanically inclined (as a kid I used to love to take my mothers appliances apart - luckily they always went back together and worked well when I was done!). I didn't know squat about tearing a sewing machine down when I started collecting and cleaning/fixing vintage machines. I learn something new with each machine.
If you are mechanically inclined, there are lots and lots of free manuals, tutorials and video's on cleaning and repairing Singer machines, especially a 15-91 as they are much sought after machines. There is also a great support group here who have helped me, and many others along our vintage journey.
So keep your eye's open for machines on Craig's list, E-Bay and goodwill shops. A hint ... E-bay tends to be higher than Craig's, but you see a broader area on E-bay and many will ship. Generally speaking ... E-bay seems to be people trying to make a few extra $$, Craig's list seems to be people trying to rid of stuff (sometimes the stuff is real crap).
And ... you might also consider what is referred to as a "15 Clone". These are Japanese (usually) made machines that were cloned after the Singer 15. Lots of times they come in lovely colors as well (Singer like Ford gave you a choice of black, black or black). Most (all?) clones will be class 15 ... but not a 91 with the potted motor and direct gear drive - they're all belt driven. Still great machines, and many will have a deck mounted push-button to raise or lower the feed dogs (the 15-91 has to be tilted forward and a thumb screw adjusted to drop feed dogs).
Take a look at some 15 clones here ...
http://www.quiltingboard.com/picture...ml#post6072223