Linen material is loosely woven and so it stretches. You would be better off using a more tightly woven fabric.
A good place might be Sears for patio dining chairs. Sears has good sales and quality patio chairs starting at $200 each. Lowes also carries good quality patio chairs at a reasonable price.
For informal parties I wouldn't recommend spending a whole lot. Walmart offers a wide variety of dining room chairs that are small in size and are very durable. My first dining set was a wood dining table and chairs cost me only 50 bucks.
You can find a couple of wooden dining chairs to match the ones you have by going to www.kettlewood.co.uk. Another good website is www.bayscore.org/diningchairs/index.html
The places to purchase contemporary dining chairs are many and varied. The best place to start would be Amazon. If you are on a tight budget, a good option would be to try eBay.
Sales on outdoor dining chairs can be found a large department stores such as walmart or home depot. You can also browse online for clearance prices on chairs.
Teak dining chairs are very nice to have for a large family, especially the really high quality ones. However, these can be very expensive, particularly if you have to buy a lot of them for the family.
I think two new chairs is an easy addition to your dining room. Since it is unlikely that you'll find the exact chairs your son broke, try for a contrast and you'll still have a good look.
The best deal on dining room tables and chairs would be to get the set for free with no charge. Though one can get a set of dining table and chairs at a very good and small price from auctions, used furniture stores or at a retail store which is having a big discounted sale.
No. Dining chairs would not be a good substitute. Dining chairs are intended to be used for an hour or two, not for eight to ten hours a day. There are many styles of office chairs that will be comfortable for you to sit in all day but still be attractive. Here is an example of a very attractive chair that would also be comfortable: http://www.lampsplus.com/products/Hillsdale-Ambassador-Office-Chair__N2973.html
IKEA is a store which sells stylish furniture at a low price. They have plenty of dining chair options in various materials and colors.
Six is probably a good number, although many sets come with only four chairs.
Square chairs would be good. Anything modern should work well, with angles and not rounded.