I highly doubt that Lowes or Menards carries Plexiglass. They probably carry a much lower quality acrylic that they sell to people asking for plexiglass. That's all people have heard of not realizing that plexiglass is a specific brand name.
All the acrylic I've seen from home improvement stores has been very low quality and I would not use it for any herp application. It warps and yellows very easily.
I get real Plexi or other higher quality acrylics from glass shops. No surprise, that's also where I get glass. Look in your yellow pages and call glass shops that don't limit themselves to automobile applications.
Even with glass shops you need to clarify what you're getting when you buy "plexiglass". They're also very used to selling acrlyic as "plexiglass" even when they carry something else. However, I've never ended up with low quality acrylic from a glass shop.
Regular glass is almost always more cost effective than decent quality acrylics.
If you want sliding doors, I'm not sure that acrylic will work in the size you'll require. It may be too flexible.
****************
Which is more cost effective for the front of a snake enclosure? I can' get plexi at lowes or menards. Where would I get glass? What thickness should I use? How much should I expect to pay? The enclosure has an opening about 3.5 x 3.5 feet, I plan on either two sliding doors, or two doors that hinge outward.