Karezza works best with female condoms. They can be ordered here: http://www.undercovercondoms.com/
Forum members give condom advice:
(Rachel) My lover and I used the female condoms early in our relationship and I think they would work well for karezza~~we made inserting it part of the fun (I let him do it as he has really long fingers)~~you will probably laugh at first because they are so different, but they work well and no worries about whether you have an erection or not!
(Clarity) I like the female condoms a lot, too. I've never liked the way male condoms feel like a one way trip, like once you get started you've got to keep going.
(emerson) Condoms are okay - even the male kind. If you have to use them, they're allright. They do require adjustments/worry and sometimes new installation of a fresh one now and then. There is a scale of arousal, from 0 to 10, with 10 being orgasm. Condoms only work if you are a 4,5,6 or higher...so you have to be hard and stay kinda hard.
(Daniel S) I was seeking rather desperately for advice since no matter how "successfully" I seemed to be practicing Karezza, I was always feeling drained and weakened afterward. ... We have since started using a different brand of condom (we had previously always used the Durex brand) and I am no longer experiencing the tiredness I wrote about. This has been a very remarkable thing, after months of such unpleasant, debilitating symptoms.
But the most amazing experience was what happened when we, in a totally unplanned way (we were out of condoms), made love without a condom. This was a completely different world - unbelievable! I had been reading about all the beautiful experiences of people who practice Karezza and had been frustrated for a long time since I had not been experiencing any of these. And during and after making love without a condom, there was a sudden click - this is it! It was so indescribably deep and sacred. It was obvious that completely different things were happening from an energetic point of view. To say we felt connected afterward would be an understatement. This palpable, beautiful, sacred energy was so strongly present. It was magical! The difference between with a condom and without a condom was like day and night. Now I can understand all these beautiful things that Karezza practitioners have been writing about, which I had not experienced until then.
Since we do not want to take the risk (and do not wish to use birth control pills), we will probably not be making love without a condom again. (As I mentioned, it was completely unplanned and unexpected). But for me it is now clear - what Mantak Chia, von Urban, and other writers have said about condoms interfering with the energy exchange is quite undoubtedly true. We are going to order lambskin condoms (it seems there is only one brand available?) and try these.
(freedom) We’ve tried male and female condoms. We’ve also tried Trojan Natural Lamb. We’d rank the lambskin above the female condom. Although the female condom might be better for karezza in terms of erection strength not being as much of an issue, the lambskin ones feel better for both of us. The lambskin ones seem to allow for greater energy transfer. We find synthetic condoms feel like rubber bands, which can push us toward O territory. That doesn’t seem to happen with the lambskin based upon our early trials. You need to hold the lambskin condom on when you withdraw. NOTE: lambskin condoms do not protect against STDs.
Natural Lamb condoms aren’t cheap. The best deal we’ve tracked down is through Bed, Bath and Beyond if your local store carries the 10 pack and you use the abundant 20% off coupons.
While we’re on birth control topic, we also use a diaphragm with Contragel. Contragel isn’t available in the US, but you can order it from some places online. It’s cheapest in Germany if you live there or happen to be there. It’s lactic acid based and smells like cookies.
As for pregnancy caused by pre-cum, the risk is lower than you may think. This excerpt comes from a 2013 article entitled "Why Still So Few Use Condoms."
The truth is that the chance of pregnancy by pre-cum is so remote that it is a statistical nonfactor. Two separate studies conducted by the National Institutes of Health found no sperm in pre-ejaculate fluid, as did a study conducted by Connecticut State University in conjunction with Princeton University . The Department of Obstetrics and Gynecology at Rabin Medical Center in Petah Tikva, Israel also failed to find any trace of sperm in pre-ejaculate fluid, and the Kinsey Institute at Indiana University maintains that "pre-ejaculate rarely contains sperm." Despite the overwhelming evidence -- some of which is readily available at sources like WebMD -- some continue to propagate the myth, maybe because they believe the end justifies the means.