I bought a replacement for mine on C/L, but by the time I changed a seal on mine, it wasn't that great of a deal. I'd just buy a new one and hopefully it'll last another 10 years.
Also, something else to check: according to the Spaguy, the drains always crack on these Calspa hot tubs. Mine's about the age of yours, I had my doubts but when I had mine empty, I checked it and sure enough the drain on mine's cracked like he showed! I ordered a new one for like $10-12. It's an easy and quick replacement if you have the water out. I had to trim the length of mine about 1/4" with a hack saw.
If you do replace yours, here's an install tip: Don't slather silicone sealer all over it like the Spaguy says because the seal will spit out as you tighten it. I went against my better judgement and did that and had to take it apart and redo. Rather, get everything nice and dry in the area, put a little silicone sealer on the threads (just a little) and a very small amount on the seal mating surface of the new drain piece (which will help lubricate it so you can tighten it more easily). Then, put the new rubber or silicone on the DRY tub surface (with both the seal and the tub surfaces dry) and tighten with a pliers. If you put the seal on the tub dry, it won't scoot around as you tighten it and you can get it nice and tight. If you silicone everything up, as soon as the seal just begins to get snug it'll spit out the seal before you can get it very tight.
The other thing that tends to cause trouble on older tubs is the heater can leak at the point where the electrodes go through the heater case which leads to the heater not working. But might as well wait till it gives you trouble.
