There are a number of decent options I've used in the past... Keep in mind that I do use a genuine stand, but as for materials, these are reasonable facsimiles:

Tatami Omote – The real deal. I've got a supplier here in Orange, CA at http://www.tameshigirimats.com/ that sells them rolled for $7.95 each (minimum 5, sold in lots of 5), or unrolled for $5.95 each (minimum 5, sold in lots of 5). They also offer them in bales of 30, for as low as $3.65 each. I've never used nihonzashi but I've had great luck with tameshigirimats and avoided shipping based on being able to pick up.

Woven Beach Mats – Walmart (or similar) puts woven beach mats (similar to this) on clearance at the end of each summer. These things cost them about $0.15/each, they sell for about $10/each during the season, and go on clearance for about $0.99 each. Cut the fabric off the ends (it's synthetic and doesn't cut cleanly), and roll.

Pool Noodles – If you're just looking to measure the cleanness of your cuts and your aim, pool noodles are great. Dollar Tree sells them for a dollar a piece if you order 32. These are great – if you aren't cutting properly, they'll flop about. If you are, you'll slice clean through. Similar and cheaper is buying bulk pipe insulation – cut to length and seal around a stand.

One tip: local instructors know local suppliers. I found tameshigirimats.com by paying attention to a local shihan. Keep good relationships with others in your area, and chat them up about their suppliers. It's good business for both of you, especially if you name drop to the vendor (Small vendors often like to deal with a handshake – being able to create a connection gets you better supplies; keep in mind, however, that you have to treat them well going forward or it reflects on both of you).