Please, please, please wait until she's at LEAST 1 yr old (even longer, if possible) -- formula should be her main source of nutrition w/ some solids thrown in. Giving her cow's milk is *not* a good substitute! Period. (I'm sorry that sounds harsh, but it is true.)
Congratulations on having nursed for 2 months - that's really, really wonderful, and many babies don't even get that chance. It sounds like you would have preferred to nurse her longer, and I'm sorry that didn't work out. I'm sure that was difficult! I, too, had really tough times in those first 2 months with getting breastfeeding established - when problems occur, it can be sooo exhausting to pump around the clock, to maintain supply, etc. - I remember it all too well (this past February/March for us).
Please know that if you are planning on having FUTURE babies, you can probably breastfeed for longer if you wish. Many moms who haven't been able to breastfeed their 1st babies (at all or for shorter than they'd hoped to) go on to successfully breastfeed their 2nd, 3rd... babies. A great resource is http://www.kellymom.com (and if preemies run in your family, check out http://www.kellymom.com/bf/preemie/preemie-links.html).
Bottom line: it's TOO EARLY to switch from formula - formula or breastmilk should be the staple of her diet for *at least* the first 12 months --> so, so important (even if the pediatrician says cow milk + vitamin is ok, he/she probably wouldn't feed this to their own baby). Personally, I'd go hungry (not buy food for myself) before I'd run out of formula for my baby. Remember, cow milk is for baby cows and not baby humans - the nutritional need of a baby cow and a baby human are totally different! When your baby is old enough to start w/ some milk products, plain full-fat yogurt is best (good probiotics for their tummies!) and full-fat whole milk until at least the age of 2 yrs old.
Please contact WIC and see what they can do for you even though you don't qualify - explain that you're in a pinch. Or talk to your pediatrician. Or ask a friend for a $20 loan and pay them back as soon as your check clears. Or even go to the local free store. It's worth it!
Best of luck to you guys, and *CONGRATULATIONS* on becoming a mother and on giving birth to a wonderful daughter 8 months ago!