Don't have verjuice or access to crabapples to make it yourself? Use pickle juice in place of verjuice with meats. It tenderizes the meat, so it's great to use on cheaper cuts, and it adds a flavor that cannot be beat. Another sub for verjuice is red wine vinegar. You don't want to use as much as you would with the pickle juice, but I almost always add a good 1/4 c. of red wine vinegar to the beef roast in the crock pot. It's always fall-apart tender!
If you're trying to make a meal more medieval, use chick peas instead of other types of beans or peas. There were no green beans in the medieval period, but chick peas were readily available. They are delicious and nutritious, and in my experience, even picky kids will eat them!
Can you eat pasta with a medieval meal? YES! The Arabs brought dried pasta to the Mediterranean back in the 8th century. One starch you cannot use is potatoes! Those didn't make it to Europe until the New World explorers brought it back. Green beans came from the Caribbean, and tomatoes are from South America! Be cautious about adding ingredients that hadn't even been discovered in the medieval period!
If you need the color of saffron threads but can't afford them, turmeric is a wonderful substitute. It won't give you the flavor of saffron, but at around $150 per ounce on average, most people can't afford to use a lot of saffron, especially when it's just needed for the golden color. The flavor isn't exactly like saffron, but no one will likely know the difference! You can also find turmeric called manjal or haldi if you're in India or the surrounding countries.
The key to good cooking from any era is flexibility and creativity. If you can't find ingredients locally, search Google's shopping section for what you could order online. Call specialty markets that you might not usually go to shop at; sometimes they have things from other parts of the world that no one else has! Take the time to grow the ingredients that you can. Herbs and veggies are easy to find seeds for through catalogs and online sources, and small gardens can be created even in apartment settings using buckets, planters and small community plots. Get in on a garden with friends, and create a medieval feast with the bounty!
Food should be tasty and fun to prepare as well. Enjoy the range of options that the modern world offers while trying to stick to at least the overall idea and feel of the food. You'll be glad that you took the extra time to make these meals, and the easy shortcuts won't even be noticeable to the eaters! They'll just know it tastes good!