The first thing you need to do is identify the make and model and, if you have it, the year that it was installed. Once you have these it should be relatively easy to locate the parts for most stoves, either from your original stove dealer, the stove's distributor or specialist stove components companies like our own, Stove Spare Parts.
Since about 2005 there have been a lot of Chinese and Eastern European stoves imported into the UK and Ireland and some of the importers and some of the brands no longer exist. So, as far the important components go for these stoves, like fire grates and baffle plates, you may have to accept the fact that your stove has reached the end of its useful and safe life and think about buying a new one. Alternatively, as a number of these stoves were copies of copies, particularly Chinese cast iron stoves, you may be able to identify a similar stove and then check the size of the components against your own to get something, perhaps with a little bit of judicious grinding, which could do the job – provided you're sure it's going to be safe.
For firebricks, heat-resistant glass and rope seals it's much easier. Again, from a specialist website like Stove Spare Parts, you can buy vermiculite sheet which is generally used to successfully replace the old refractory clay firebricks which are expensive and delicate to transport. Vermiculite is very easy to cut with a hand saw or electric jigsaw, using the old brick as a template. For heat-resistant glass all you need is the exact dimensions, or if your glass was arched or shaped a precise paper or card template (which must be posted and not emailed). Fire rope comes in a number of diameters and it won't cost too much buy the diameter you think will do and, perhaps the next size up or down as well, and then to check which one works best in the stove's rope channel without putting too much pressure on the stove door hinges.
If your stove is a relatively new and popular British, Irish or Scandinavian stove then replacing a part becomes much easier. However, you should note that even with these stoves the spare parts are not like car or washing machine spares, where you can generally quickly buy a replacement anywhere. It can often take time to source a replacement stove part, even from the very best companies (some are better than others) so our advice would be, at the first signs of a component failure, start to track down a replacement right away so you're not left with an unsafe or inoperative stove. If you can do this before the start of the heating season in September then this will make your life so much easier.
PLEASE NOTE: Never operate your stove if you think that it may be unsafe or where smoke or fumes enter the room. Always seek professional advice, have your stove serviced annually and regularly check your Carbon Monoxide (CO) detector.