Hi! Sorry about your boxer's issues. I love the breed, but they are definitely notorious for allergic issues.
What kind of diet did you put her on? I would suggest trying a grain-free diet. Most dog foods that you can buy at a grocery store will not be grain-free, unfortunately. You can usually find quite a few choices at your local pet store though. The key is examining the ingredients and avoiding things like: wheat, cereal by-products, corn, etc. You want meat to be the first ingredients (and not meat by-products). It does take a little extra effort and the brands tend to be a little more expensive, but it's definitely worth it if it helps your dog's allergy issues. Candidae is a pretty affordable brand, but there are tons out there.
Also - what are you cleaning her ears with? There are products for sale that you can use, or you can use a 50/50 mixture of vinegar and water. Never clean her ears out with just water, it can actually increase the chance of infection. Anytime you give her a bath, or she plays in the water, make sure her ears are dried so no moisture builds up.
Good luck! Allergies are a pain, and it's so miserable to watch your dog be uncomfortable :(
If she is getting yeast between her toes that does sound like she is allergic to something.
Maybe first try to just change her dog food and remember to do it by adding the "new" a little at a time to the dog food she is now eating and then do away with it completely and keep her on the new food and see if there is change. You probably know switching dog food in too quickly causes diarhea (sp?)
Good luck,
MO
oops, I just read you already tried switching dog food. Well the only thing to do is somehow get her the test even though you say its expensive...Put it on a credit card or borrow the money from a family member. Its not fair that she would suffer with this and if she is licking her paws it is bothering her..