They would have the M42 x 1 screw lens mount.
I've had a quick look on eBay. There are 7 for sale (i.e. 'Buy it Now'), ranging in price from £4.99 to £24.99.
Most of them are made by AICO, though there is a Helios at £14.99 and two Vivitars, one at £12.00 the other at £24.99 (they look identical to me, but I didn't look at them individually in detail).
As discussed, using teleconverters is a compromise, so it is best to go for the best. If it was me I would consider the Vivitars.
Regards