Start with the easier large balls and work your way down to the the smaller ones as you get batter. This is a good example of why no one ball will suit all needs. However, unless specifically noted, I generally talk to contact juggling here, not body rolls, multiple ball patterns or anything like that. You are certainly not limited to balls you find at juggling vendors. Acrylic, silicone and stage balls are just a start.
Pool balls work nice in that they have great weight but are a bit on the small side.
Lacrosse balls are not bad, and also pretty cheap. Hollow glass christmas ornaments are too light and prone to fly off your hand. Just beware when getting a ball too small for contact juggling, it makes the rolls harder to control. You do not want to use anything so small that your hand dwarfs the ball. But, your choice should depend on what you feel comfortable with and what type of movements you plan on working on.