I know you said you don't drive, but I buy a lot at garage sales...it comes in handy for piecing, and you can't beat the price! Tell your friends/family to watch for you when they go..tell them your top price per spool you're willing to pay...and set them loose! I got an entire shoe box chock full of thread at one sale for 2 dollars..they might enjoy surprising you and getting you little what you want..in my experience..people that go to garage sales a lot like the hunt! If you give them an item to hunt for, they will deliver!
I also watch ebay...but that hasn't been panning out too well lately because the cost of shipping kills any deal I might have gotten.
Connecting Thread is cheap enough..but the lint is unbelievable! I have about 20 spools that I just cringe when I think of using it! It's fine on the older machines, but the new computerized machines do not like lint!!!