Seriously... motorcycle boots. Costume boots may look cooler but they won't last an event of hard use unless you shell out a few hundred dollars for high-end, well-made costume boots. Motorcycle boots will last ages and you can get a pretty good pair for $40-$60 (mind you this was as of five years ago so I don't know if the price has moved up more twards the $60 end). They look sufficiently period, have good tread, and are durable.
Never never get costume boots from a costume store or wholesale costume site... the only costume boots generally worth anything are custom things like you see with (some! Not all! Do research) renfaire vendors. Most costume boots are made for halloween or party wear, which is to say - worn a handful of times, indoors, very light use. Not to be walked in outdoors on trails for hours and hours and taking a heavy beating.
Durability, safety, and comfort are really the higher priorities with LARP footwear. Appearance just needs to be period enough. A nifty appearance is worth nothing if you injure yourself at the first fight of the event, or if you find yourself without footwear halfway through because your shoes fall apart.
(And I say this as someone who plays more for immersion, roleplay, and character than for combat. I learned the hard way about the importance of good hardy footwear my first season as street cast at a renfaire - got $100 boots from renboots that fell apart halfway through the season. What a freaking waste. My motorcycle/engineer boots on the other hand have lasted me two years of wearing them every day and at LARP. Well, well worth it.)
