Normally RAM comes with a heatspreader only if it's likely to be needed, mainly sticks designed for overclocking. RAM sticks without heatspreaders are generally inferior, not because they lack a heatspreader but because they're designed for purposes that wouldn't require one. You can buy heatspreaders for existing RAM,
here, for example, but it probably won't improve your performance unless you have high quality RAM already and are looking to overclock, in which case it may help system stability.