Once a year seems a little often, One of my backs has not had the trap replaced since i've owned it, and that is going on 5 or 6 years now.
I found that the cost of buying them directly from Hasselblad was about the same as the ones I saw for sale on the bay (and less than some I saw). I called them up, (Hasselblad USA), gave the person my cc# and two days later two kits were in my mailbox.
As for keep the slide in, or not, I've read that keeping the slide in keeps the seal compressed and eventually leads to light leaks. FWIW, I only have the slide in when I have a reason to remove the back. As noted, there is no issue with loading without the slide in place. The only reason to put the slide in is to either remove the back or as a way to lock the shutter to keep it from tripping accidentally.
If i´m not mistaken the "once a year" deal was recommended from Hasselblad but then again this was back in the day when photographers did not do any servicing themselves and more photographers had more money to spend. I use a strip of electrical tape over the slot a period before i change mine. Shoestring budget :-)