You can get one of those prepaid debit cards. My son bought one for $5 at Walgreen's and preloaded it with funds so he can buy online. You can get the same card for free at Greendot.com. There's also a prepaid card at rushcard.com. You can get your paychecks direct deposited at Rushcard and you never have to go to the bank. You'll pay a one time fee. When the card expires, they automatically send you another one. Russell Simmons, the king of hip hop, started the company just for people like you with no credit cards and for low income people to allow them to buy online and not have to worry about stolen checks in the mail. This is a debit, not credit, card so you can't go over and get in financial trouble.
As for yarn, you can make one big trip and stock up for a while. I don't know what type of yarn you like. Last fall, I bought five different colors of acrylic Caron One Pound yarn for around $40. I made hats and gloves for my adult sons and their roommates, plus charity mittens. I'm still using that yarn and just now coming to the end of it. They come in solid colors. What I like is the 10 ounce multicolor skeins at the end of this page. These multicolor skeins match every One Pound color they have. I've gotten so many compliments on the things I've knitted with the Jetstream multicolor 10 ounce. The colors are gorgeous.