From my experience, (in the UK), they are quite scarce and one tends to only come up on eBay every couple of months. It is more common to find them installed in machines but then there is the issue of whether to rob a working SE/30 of it's card or just use it in that machine.
When the cards do come up by themselves they tend to sell for a moderately expensive price, (about the £20 mark from what I've seen), so it really depends if you think it is worth it. It might possibly be better to get an ethernet to SCSI adaptor such as an Asante EN/SC, since this could be used with the SE/30 and other Macs as well.