6 Deck discard tray: $8.00
http://www.amazon.com/Deck-clear-acrylic-blackjack-discard/dp/B000G623BW/ref=sr_1_3?ie=UTF8&s=sporting-goods&qid=1207690715&sr=8-3
13 decks of cards: $13.00
(Dead link: http://www.amazon.com/Bakers-Dozen-Decks-Different-Casinos/dp/B000G9Y3GC/ref=sr_1_13?ie=UTF8&s=toys-and-games&qid=1207690798&sr=8-13)
Cut Cards: $2.00
(Dead link: http://www.amazon.com/Professional-Quality-Casino-Card-Poker/dp/B000ZQDUW4/ref=pd_bbs_1?ie=UTF8&s=sporting-goods&qid=1207690880&sr=8-1)
Practicing with real, live cards in your hand: Priceless (and cheap)
ok - in all seriousness, im just going to paraphrase a method i read in Blackbelt in blackjack many moons ago which helped me get my estimation down (in addition to using CVBJ for the first few hours of practicing estimation):
take 6 decks (or 1 or 2 or 8), pile them together, and then number them in the corners with marker from 1 through 312 (or 52 or 104 or 416). stick them all in the discard tray
practice sticking the cut card into the stack of decks sitting in the discard tray, first aiming for 1D increments, then half deck increments (and then quarter deck increments if you so please). in other words, try and cut one deck from the bottom, then 2, 3, etc. then try 1, 1.5, 2, 2.5, 3, etc. take a look at the number written on the card right below your cut card to check your estimate. if you are trying for 2 decks for instance, you should see 104 on the card right below your cut card.
once you can do this, just randomly remove varying chunks of cards from the top of the stack and estimate what you see sitting in the tray. take a look at the number written on the top card of the remaining pile to check your estimate.
with a little practice you can get accurate down to quarter deck if you choose. quarter deck is recommended for single/double deck while half deck is what i recommend for shoes (though plenty of ppl use full deck estimates until the last deck or two of a shoe).
finally, if you buy enough decks as previously suggested, you can just create permanent piles of 1 deck, 1.5 decks, 2 decks, etc. just keep them around your house/apartment and keep staring at them and your eyes will callibrate.
in all honesty, after only a day or two of staring at these things i bet you can pretty accurately estimate them to at least a half deck.