Let's talk abut Shoes and LARP
Someone once told me that the first thing they notice about a person is their shoes. Do they match in style, color, and cleanliness? What you put on your feet is just as much a part of your costume as the vest, coat, hat or other more "eye level" pieces.
This isn't to imply that you can't wear sneakers or chucks. You just need to do it INTENTIONALLY. Are you playing an Anarch vampire who wears gaudy tshirts? Great, make sure those shoes are just as offensive. (Old sneakers with a suit are a great way to accomplish this.)
This isn't just about form, it's about function as well. If you're playing a four hour parlor larp, your options widen significantly as opposed to hiking over a whole weekend where the terrain and weather may provide challenges.
I'll break this into two parts: Indoor and Outdoor shoes.
Indoor Limited Wear Shoes (ie I'll be sitting and RPing)
Fancy, sexy shoes do not have to be uncomfortable. Inserts, ankle support, and smart choices can always be made that will help you enjoy your time looking fancy.
If you do not have the ability to wear heels indoors, consider ballet slippers as an alternative. Still cute, still fashionable, but not as nasty on ones feet.
If you are going to wear heels consider getting wide or chunky heels to save your feet and lower back some grief. Still get that lift, without all the pain!
Indoor shoes are easy to shop for on amazon, zappos, or your local thrift store.
Outdoor "Weather Oriented" Shoes
Outdoor shoes are a different story. You need to consider their durability, weatherproofing, and what socks and inserts may need to be used.
If you're going to buy online - READ THE REVIEWS. Seriously.
If this is for your first time out, buy something less expensive and instead invest in good socks and inserts first. Look at the weather report, determine if it's going to be wet. If there's weather in the future, consider waterproofing the shoes, and wear something that goes above the ankle. Trust me on this, I've seen people's shoes be eaten by mud puddles....
If you know you're committed to a long term relationship with the LARP, do yourself a favor and spend at least $60 on a GOOD pair of boots. You'll likely have them last for several years, and they will protect your feet making your experience that much better.