I replaced the gaskets on my riteway stove with 3/4 in rope that I obtained from a local hardware store. I also believe home depot sells stove gasket kits. Also, my stove is not near airtight (as no stove is airtight). In fact by me adding the secondary nurn tubes more ar is pulled into the foirebox above the coal fire. I do not cover the input to these pipes up when i burn coal because i do not care if there is overfire air when burning coal. This stove also has the secondary burn chamber that has holes in that also for further burning. The only thing the secondary air does in my stove when burning coal is slow the air draw down through ther coal bed. Which is ok by me, if i need hotter i just open the thermostat slightly more. Also, I found that I had to drill a new hole in the input air plate and move the chain an inch further away from the hinge, this helped me control the fire when burning wood much better becaude the flue temps are so much higher when burning wood that it would swing that air control open and closed all the time. With the hole for the chain 1 inch further from the hinge the flapper did not open and close as drastically. This was more important for wood but i guess if your stove's ash pan is leaking your flue temps could be very high also.
Just a thought,