I would recommend buying stronger magnets, like from K&J Magnetics:
	
	
		
			
				
			
			
				
				
				
					
						
							 
						
					
					www.kjmagnetics.com
				
 
			 
		 
	 
Depending on your plans, they make waterproof magnets, rubber coated, etc. You can buy any strength, including truly dangerous ones with hundreds of pounds of pull force. The folks at K&J are also pretty geeky and cool about helping you figure out what you need for your project.
You can also get them cheaper (and coated) from Amazon, etc.
Restricting yourself to whatever you find in pumps seems like not the right approach to me unless I’m missing something.