What sort of pump you need depends on what you are going to use it for, and you need to think about whether it is worth buying your own. If you're just doing top-ups, you only need a basic pump, but to replace displacers you will need a Vacuum pump. These pumps suck the air out of the empty system before pumping the fluid in. Most units have a pressure gauge but unless you're modifying the system, trim height is more important than pressure.
The original pumps made for BMC were made by Churchill. These are the 'Dalek' units with handles coming out the front. When Churchill went under, MG Rover sourced pumps from Liquid Levers in Scotland. The other popular manufacturer is Alba, who make a round one in red and gold.
Although a Hydrolastic/Hydragas valve is an ordinary schrader valve, you can't use a normal schrader coupling as the very high pressure fluid will leak out when fitting. A low loss connector and is designed not to lose fluid when connecting and disconnecting (hence the name). These connectors feature a T-shaped handle which you wind in to open the valve once you have connected the connector. These are available from Liquid Levers or eBay.
Some people have found successful ways to make their own pumps, obviously use at your own risk.
As with parts, eBay is a good place to pick up a new or used pump.