Infants: they pay 10% of standard fares, and do get issued boarding passes (with your seat number) virtually everywhere. Only discount carriers take infants free, and do not give them boarding pass (I think no insurance either).

Usually it NOT worthy to get ticket issued with travel agent, since agent has to charge you a fee that is not 6-10% of these 10% but something fixed at min - like 40$ more than airline real fare. Hence in most of the cases it is best to arrive at the airport 10 min early and buy ticket there.
However, sometimes there are 1m, 3m, 6m, 12m infant tickets, that are not available at the airport - but only in city office of airline or agent.
So do your own homework and check.

For the note: infant age is counted as of date of departure. If you buy 1yr return (infant tickets have been freely changeable in 90% of cases) and your kid turns 2 in-between, no problem to use the rest of your ticket. If you use one airline on regular route, it can be prety handy to accumulate several tickets like that close to kid's second birthday.
Theoreticly kid's ticket is related to particular adult, but that adult can be changed by calling the airline.

Another note: As soon as kid turns 2, you not only start paying 66-75% of adult fares (and full penalties related to changes etc.) but you can also collect 100% miles on those painfully expensive tickets (kid still insists on travelling on your lap till 3). Those miles can be used by adult - which means you pay less for your airmiles as return for the inconvenience to both pay seat and have bigger munckin on lap.