I found a source to purchase the polyethylene plastic sheeting to cover mattresses which is cheaper than buying the mattress covers. As crazy as it sounds, I found it on Amazon.com!!!
Here is a link to one size:http://www.amazon.com/6-Millimeter-Polyethylene-6ML-CLR-12X50/dp/B000H6VQ00/ref=sr_1_3?ie=UTF8&s=hi&qid=1203470067&sr=1-3
They have different sizes and quantities. If you are interested, just put polyethylene 6ML in the search box on Amazon and you will see your choices. I know it sounds like a lot of plastic, but if you have others to share it with, it is REALLY cheap. I already have several families I know who are going to split it with me.
Here is a blurb about wrapping the mattress (from:http://www.cure-guide.com/Natural_Health_Newsletter/Baby_Bedding___SIDS/baby_bedding___sids.html):
HOW TO WRAP A BABY'S MATTRESS FOR SIDS PREVENTION
Instructions written by James Sprott
The advice to wrap mattresses applies to every mattress on which a baby sleeps (except a BabeSafe mattress) and includes: adults' mattresses; mattresses of other children; and all mattresses made of or containing natural products such as sheepfleeces, goatskins, kapok, tree bark, coconut fibre, etc.
The most convenient way to wrap a baby's mattress is by means of a BabeSafe mattress cover. As an alternative, parents can purchase polyethylene sheeting to make their own mattress wraps. If they select this option, the following instructions apply:
1. Use thick, clear (not colored) polyethylene sheeting, available in the paint section of your local hardware store. The thickness of the polyethylene must be at least 125 microns, or 5 mil. On no account should PVC be used for wrapping mattresses.
2. Place the polyethylene over the top of the mattress and down the ends and sides, and then secure it firmly beneath the mattress with strong adhesive or duct tape.
3. The polyethylene should not be airtight on the underside of the mattress. It must be airtight on the top and sides of the mattress.
4. It is imperative to use the correct bedding with a BabeSafe mattress or BabeSafe mattress cover or polyethylene-covered mattress. On top of the polyethylene place a fleecy pure cotton underblanket and tuck this in securely. Then make the bed using sheets and pure woollen or pure cotton overblanket/s.
5. Do not use any of the following items in your baby's bed:
Any form of moisture-resistant mattress protector
6. Proprietary mattresses and mattress covers must not be used unless they carry the Campaign against Cot Death logo or are accompanied by a certificate of analysis showing that they contain no detectable phosphorus, arsenic or antimony (lower limit of detection 0.001% = 10mg/kg = 10ppm).
For more information about baby mattress issues read the articles at the healthychild website.