I don't have the luxury of flying to Belgium (one day, the wife swears she'll let me go for our 10th anniversary) so I generally end up placing 4 - 5 orders per year. I use 3 sites - Etre, Belgium in a Box, and Beergium. However with BiaB's recent crazy pricing for Cantillon ($33 for Rose 75cl!) I might stop using them. Here's my advice:
Etre allows you to have bottles held in warehouse so you can assemble a large order over 3 months or so (they force you to ship out at the end of every quarter - dates are posted on their site). Just select "Reservation" instead of shipping. Very helpful. Just keep monitoring their site and you'll get lucky to see some Cantillon regulars and semi-limited 3F stuff pop up. Then fill the rest of the box with Vat 109 and Fantomes to maximize shipping. You pay 78 Euro for up to 11 75cl and one 37.5cl bottles in one box.
Beergium prices are cheaper than Etre but their shipping costs are higher so it's generally the same cost. They usually have a decent stock of regular 3F gueuze and kriek. Sometimes they get in A&G and rarely a Cantillon or two. They have great customer service (I prefer to use FB to message them).
BiaB is good too but this past week their prices on Cantillon shot up like crazy, so I'm not sure what's going on there.
Finestbelgiumbeers has higher than normal prices (still lower than Lambicworld) so if you're really fiending for some RdG, you can get some from them for under $29 I suppose.