Hi, IMS! Glad to provide some info for you.
Re moss: an inexpensive source of moss is Home Depot's Garden Shop. This good-sized bag is only $4 and will last quite a while:
http://www.homedepot.com/p/Mosser-Lee-4 ... /202301542
Re bubblers: they work equally well in FW and SW but the SW bubbler will need to be replaced more frequently due to clogging. This is not a big deal as small airstones run about $.50 each.
Re shell shops: I, too, have a 29g and have set up two shell shops: a suspended one made from a suction cup soap dish from Walmart for small shells for my little guy and another made from a plastic basket set atop another overturned basket (doors cut out to make a hide) on ground level for the larger shells for the other three crabs. Most crabbers choose to place the shells in some type of container to prevent the constant burial of shells set out right on the substrate. The picture below shows both the large and small shops in my 29g.
Re shell sources: as you still have a number of small crabs, check out craft shops (Jo-Ann, Michael's, Hobby Lobby) for bags of shells (at Jo-Ann, they're in the Floral department). These run around $6 and contain an assortment of wearable green turbos, babylonia and nice flat scallop shells for food serving. As the bags are mesh, it's easy to see the sizes and condition of the shells so look for a bag that'll give you the most usable assortment for your crabs. The
Shopping Resources page (Navigator box on the Forum page) lists several reputable shell shops; Naples Sea Shells provides the size of the shell
and the width of the opening so they're always a reliable source; MyGann on ebay is another vendor that comes highly recommended by HCA members.
Lastly, re the number of crabs: as I mentioned above, I have a 29g with only 4 crabs - 1 large, 2 medium, 1 small - and I seriously wouldn't add more. Given that you have 7 crabs, medium and small, in your 29g, you're really past capacity at this point. To accommodate all 9 of your crabs, keep an eye out for a used 40B or even a 55g on craigslist or try to pick up a new one the next time Petco has a $1-per-gallon tank sale.
Hope this helps

.