Many forklifts run off of batteries for at least part of the power. It can make it a lot more efficient and cheap in terms of gasoline usage. The problem is replacing them. You need to take proper safety precautions when doing it, and you need to make sure you get the right battery to go with your model of forklift.
The first thing you're going to have to do is actually get the forklift battery. You can buy this online, get it from the manufacturer in many cases, or get it from the dealer who sold you the forklift if you've got one. Make SURE to check the parts number on the existing battery and flip through the documentation or manual to be sure of what you actually need. If you install the wrong battery, you could damage the machine or just waste money and time shipping parts back and forth.
There's a good online store here that sells parts for most models. You do need to know your exact number to be able to order though, and it's not a site that's friendly for browsing around trying to find the right part. There isn't much detail on what the parts are besides their name, which can be a bit of a hassle.
Another surprisingly good place to get forklift batteries is on Ebay. They've actually got a pretty big selection there:
First, you need to have your employees recognize that forklift batteries are an electrical hazard even when they aren't hooked up. You have to treat them like you would any appliance or machine - no spills, no rough handling, etc.
Second, remember that they can emit gases or chemicals if punctured. Sometimes gas comes out when you're charging them as well - this means NO SMOKING and no open flames around them. That's a recipe to get blown up. It also means you don't charge them unless it's in a well ventilated area. If you do it in a confined space, the gases can build up and you are at greater risk.
Third, some forklift batteries are huge. They can way several thousand pounds on bigger models, depending on the battery. Remember to adhere to proper protocols that you would for lifting and moving any heavy object.
Fourth, keep neutralizing agents around at all times, and explain how to use them to your employees. Batteries contain battery acid. If there's a spill, then use the neutralizing agents on the acid and clean it up in compliance with local regulations.
Fifth, if you're handling a forklift battery you need proper safety equipment. Goggles, aprons, face shields, etc. will all reduce the risk of an acid splash. You should also have an eye shower nearby (OSHA requires it to be 10 seconds away). Safety equipment is a federal requirement for handling these, so don't skimp on it and risk getting busted and fined. If you don't treat safety seriously, your employees won't either - and that will mean a much greater risk of injuries.
You can find detailed instructions on setting up a forklift battery changing station here.