The o nly other source I know of is this one. They normally have a minimum order value $100, but you could try writing them to see if they will make an exception. Good beta / gamma sensitivity and my best end window alpha detector. LND 712 - The gold standard in end window alpha tubes. It's tiny, but may be good for hunting Uranium glass.
122 might be a good ratio for uSv to CPM. In my tests this tube is about 70% as sensitive as the SBM-20. This is a nice tube for compact projects. SI-29BG - (specs: here) Good, smaller, cheaper, substitute for the SBM-20. (I solder the wires to it like this.) 10/12/21 - Another user found a conversion factor of 575 was closer to a calibrated measurement. A user (Justin) said that a CPM to uSv/h rate of 321 gave a similar uSv/h as his GammaScout. (I like the way it looks too.) However it's similar to the SI-29G on the other samples. This might be the best tube I have for background. SI-180G - Very sensitive to background (84 CPM) but less to other sources. It is reported that a good CPM to uSv/h conversion rate for this tube is 153.8. There is a discussion about it's light sensitivity here. It's quite possible this tube is light sensitive which would explain the initial high counts. Although I once got high readings, it now seems similar to the SMB-20. Claimed to be more sensitive than the SBM-20. The STS-5 is pretty much the same tube and may be cheaper. This is still one of my favorite all around beta gamma tubes. Lower priced, and more sensitive to beta and gamma than most. SBM-20 - The quintessential Russian tube. Performance of each is on the comparison chart that follows. When possible, I added the link to the specs and added my own impressions of each tube. In fact, it's limited to only the tubes I've tested. This section should help you get a better idea about how some GM tubes perform, and may help in selecting a tube.