I used Pellon 911ff and the cheapest way to buy it was through Walmart.com and getting their free shipping to the store neatest to me. As far as the slinky jersey, I'd say do a practice block, including the quilting. Also remember that quilting through those giant numbers most of them have is a drag. Meaning skipped stitches, problems with tension, etc.
One thing i found was that some of my shirt squares came unbonded during the wrestling match we had during the quilting process. Not sure if it was from them having built up fabric softener in them, me not bonding them well enough or with heat that was not high enough or what. I confess I did not follow the directions exactly. They said to use a damp pressing cloth and I was all, "Ain't nobody got time for that!" which ended up costing me more time in the end. Haha, famous last words!!