Need a good "under carpet" speaker cable

Need two flat 25' runs for my B&W Matrix 805's in the rear of my Theater. Don't want to spend a lot of money, but want good quality. Any help is appreciated.

I can recommend some stuff that's like $15/ft, ultra high purrity 6/9's copper, sounds pheonominally good infact, but it's kind of an industry not sure I should spill the name
Audio quest makes some stuff however, that's at least 4/9's copper, wish sells for like $6/ft, and sounds pretty descent otherwise.
I would basically find the highest purity copper flat cable you can find, by checking manufacturers website otherwise, and contact someone about availability and price.
Good luck
I would look at the Nordost line of cables, they make excellent cables at all price points. Good luck.
I'll second the Nordost line of products. I use Blue
Heaven which is probably a bit expensive but they also
make cheaper cables.

An ideal cable from a visual/low profile standpoint would be 47labs OTA cable kit. This cable costs $600 for 50meters of raw cable. Hence, this would provide 12.5 meters of stereo cable. If you can use slightly less length on the speaker cable, then the kit allows you to make three sets of interconnects as well, in a very interesting way - using bare wire for the complete connection (no metal plugs etc). The cable is very good (check out the audiogon threads on it). It is also only .4 of a milimeter thick!!, but with plastic outside is a little thicker, but still about as unnoticeable as a cable can be. Some threads say it may be the best cable bar none. I own the cable and IMHO it is not even near the best there is but it is very cost effective. Good luck.
Try Alpha-Core Goertz in copper or silver. High-quality and flat. Check reviews.
If you are going under the carpet, then I assume you would be installing new carpet?

If the floor joists run parallel to the direction you are intending to put in the speaker cable, why not cut a few access holes into the floor?

You can ask any carpenter how to do this and still keep the floor in good shape. I have seen it done with osb and with plywood floors.

You do not need flat cables then, any would do.
I'll second the recommendation of the Goertz "MI" line of speaker cable. They offer 3 models which vary in width. Goertz is one of the real values in high-end speaker cable and interconnects. Their products are grounded in actual science, not "wire mumbo-jumbo". I suggest you read the excellent material on their Web site:
I'd recommend the Goertz as well, good stuff but it may not fit your budget. If you'd like something inexpensive QED makes some decent flat cables at roughly half the price of Goertz. The QED stuff strikes me as more rugged as well. In your set up (long runs add up, and it is only for rear HT speakers) I'd save some money and go with the QED. Jeff