glass for sharpening

[RodB]

Theres usually a glass supply for custom windows etc in most towns... I just called up the local one and got a 8"X16" x 1/4" piece of "float glass" with a nice bevel on all the corners for about $15.... be sure you get float glass cause its really level, plate glass is not. I also  bought a ceramic tile from Home Depot about 12" x 12" and it works too. Just hold a good straight edge up against it to make sure. 

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[Anji12305]

Most kitchen counter builders will sell a sink cut out for little money.
Not as portable as plate glass, but reliably flat.

[RodB]

Any glass supplier can cut and bevel the sharp corners of a piece of glass... you should get at least 1/4" thick, 1/2" is better. Make sure its "float" glass, not safety glass... float glass is manufactured in such a way that it is extremely flat. 

You may need a honing jig to produce a 25 degree bevel on your plane iron for the first time.

If you get a piece about 10" by 24" you can use standard 4" wide x 10 in long sheets of wet/dry 3M sold at Home Depot...you can use a 3M spray adhesive and stick down 5 different grits to sharpen with...  220, 400, 800, 1200, 2000...  (resharpening when using you can just hit the 800, 1200, and 2000) 
 Be sure to follow your sharpening with a leather strop (suede side up of standard leather  contact glued to a piece of wood 3/4" thick 3" x 10". 

 Apply Strop paste to leather... https://www.sharpeningsupplies.com/Green...wAodmCsDLg

see following video  on sharpening by Paul Sellers... on advantages of using a leather strop with polishing compound  after sharpening. 

https://youtu.be/vvTcReENk9g

The following link on restoring a plane covers all facets of setting up a plane... well worth watching. 
https://youtu.be/RYyV6IUpsYk

[Axehandle]

I found a 1x3 and 3/8" thick piece of tempered glass that was one of 4 pieces used in a shelf. I honestly don't worry too much about how "flat" it is. It is flat enough. Woodworkers always think they are machinists.

[Malix]

I'll second the small 13 x 9 granite surface plate from Enco or similar.  These are thick, stable, and ground flat to machinist tolerances. Even the cheap ones will be more than flat enough for woodworking tools.  Unlike glass or thinner counter top material these will stay flat not flex and will last lifetimes if you treat it carefully.   Been using one for years.  Highly recommended.

Search google for granite surface plate. then  hit shopping. then lower the price to max $50 bucks.  I saw some around 32 to 35 $.