Bubble Jet Set is absolutely the least expensive way to print permanently - and machine washably - on fabric. i've been using it for years. they even make a second, special formula for the newer HP inkjet printers. (when they "improved" the HP ink, the new stuff didn't work nearly as well with the original BJS formula. go figure.)
the lowest price for BJS can usually be found at dharmatrading.com
the manufacturer also produces other products useful to those who want to print permanently on fabric - including a variety pretreated fabrics. i haven't tried the pretreated fabs yet, but if the quality of BJS itself is any indication, i expect they'd work very well. they are also more reasonably priced than any other similar product i've seen on the market.
www.cjenkinscompany.com