Is this popular to find in your everyday grocery store? I'm looking to do a quince, manchego cheese crostini but can't find this ingredient. I just learned of this combo recently so not too familiar with quince.
Quince are also available around now from farmers' markets. Yeah, they're pricey, but you can actually use the whole thing -- I took the skin and core and seeds and boiled them with water to make pectin.
Membrillo is easily ordered from Spanish food Web sites, like La Tienda and Despaña. Since it's highly concentrated, a little cube is enough. And since it's a form of preserve, as long as you store it carefully, it will keep pretty well for a long time.