Having a large cup size can be extremely frustrating, especially when the size keeps changing. When I was eleven years old my mom took me bra shopping for a training bra...none of them fit...I was a 32C in a real bra. When I finished growing I ended up a 32D, and stayed that size for a long time. A 32D bra is a hard size to find, but I managed....we had a Fredrick's of Hollywood at the local mall and they carried bras in my size that were pretty inexpensive. Then, thanks to Rispidal my boobs grew another two cup sizes...by the time I came off it I was a 32DDD....despite only gaining 5lbs.
Every time I lose or gain a little bit of weight my cup size changes. I eventually lost some of the Rispidal boobs, but not completely. A slight change in body composition (fat vs. muscle) can make a big difference in my breasts. So for that reason I constantly need new bras.
I used to get my bra size checked if I notice by bras are not fitting right after a while, but now I pretty much know how to adjust my size depending on how poorly my bra is fitting, and I only change the cup size, never the band size. I would probably need to gain 25-30lbs to go up to a 34 band, but a tiny increase in body fat (and loss of muscle) that a scale can't even measure can cause me to go up a cup size.
The band of the bra is where most of the support comes from. So it's really important that the band fits right. The straps shouldn't fit tightly, just stay up and not fall off your shoulder...the band of the bra is where the suppost comes from (hence why a strapless bra is possible). Although straps should be adjustable so they don't fall down.
For finding inexpensive D+ cup sizes you could try to wait until a department store has a sale, or try an off-priced retailer like TJ Maxx (I think it's called Winners in Canada). I've found some bras at stores like that, even in my freaky sizes. But sometimes I'm willing to spend more to avoid the mess and aggervation. Also there is always Walmart...but there are sort of limited in the sizes they carry...I'd love to be a 34C so I could shop there....