Background: I purchased my Silvia 10 months ago and have used a LM basket for about 6 months.
1. This is the only place I am aware of (but have not looked elsewhere). There service is excellent. I placed an order on Wednesday at about 3.00pm and received the delivery at 9.30 the next day (Sydney - Melbourne no less).
2. I think they only have the one type which has a ridge. Dont quote me on this but I believe 300013 is the original from LM (at least a direct copy) and 521994 is a general basket which fits a variety of manufacturers. Have a look at this link which appears to be the same organisation but with a different layout and information. http://www.jumbocoffee.com.au/accessories/baskets.html
Note the 18gm basket doesnt fit Silvia’s portafilter, however after I make a small adjustment to my naked portafilter I think it will.
3. I use part 300013 and its fits like a glove, no problems.
4. In my opinion the LM basket is a little deeper and allows more coffee into the basket, it produces more crema and a creamier mouth feel. On the other hand it does tend to let some coffee grounds into the cup.
I hope this helps