This is a handy and cheap alternative especially when tomatoes are off-season. This 150g pack is usually sold for 35Php. There are other brands, but so far this has the best quality I've used-- it's thick and the flavor is well concentrated.
If you don't have this, you can use canned tomatoes instead. Fresh tomatoes are good too, but I'd suggest boiling the tomatoes longer to really squeeze out the juices and perhaps add a little sugar because fresh tomatoes tend to be a bit more sour than canned ones, based on my experience. ;)