There is no point in showing a lame horse in halter. I would get his leg looked at and see if it can be treated before worrying about showing.
Does he jog willingly under saddle? Does he lead next to you on a loose shank when leading at a walk?
To find show tack, you can find decent deals on line if you just want something for low level shows. Even for smaller shows, a halter with "silver" is the norm.
For yourself, at lower lovel shows, jeans will work but pressed dress/show pants are better. Don't forget to have a properly shaped hat. For your upper body, you can go with a western shirt, but to be taken seriously, a good jacket is a worthwhile investment. EBay has a lot of sellers with reasonable prices.