You can do a pretty decent job with a ziplock and a water bath until you can afford one.

Put the food in the ziplock and submerge it in a tank of water up to the top of the bag while its still open. Then seal the bag while the lower part is held under water. Helpful to have an extra set of hands if you have someone available to seal while you hold the lower part under water.

Obviously not as good, but it does help.