I got two good 90 degree clamps at WoodWorkers Warehouse. However they have since gone out of business! There must be a Woodworking store in your area, or even a Home Depot or Lowes that will have the clamps you need.
United sells them as well, but to get them in time buy local!
Don't get the cheap little flat vise, you want one that has a base and is rock solid. Not hard to find, do a search for Stanley 400 vises if you need a picture.