Stores like Bed Bath and Beyond or Home Goods will sell chair covers. Depending on chair style you might be able to make chair covers yourself if you can sew. You can also try looking in furniture stores. They will carry chair covers as well.
You can purchase chair covers for dining room chairs at Big Lots, Walmart, Target or even Ebay. If you are good at sewing you can measure and sew them yourself pretty easily.
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
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.
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.
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
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.