I'm assuming you're going thru insurance? I've ordered stuff from American Allergy -- filters, neb cups, compressors, but didn't run it thru our insurance. For the most part the respiratory therapy person at our local clinic gives us bags of neb cups when we visit or if we call him up and ask.
I ordered a spare compressor -- Devilbiss from Comfort1st.com, but there are other sites. I didn't run it thru insurance 'cuz last time we did the pari proneb from Apria health care cost $356, when they were available on other sites for $70-90. I chose comfort1.com at the time 'cuz they had free shipping.