You could try the atari workshop
( http://www.atari-workshop.co.uk/ ) ... its the first place that sprang to mind, and the only place i can think of that still operates!!
Perhaps you can find cheaper tho' ... the "Marpet Xtra-Ram Deluxe 4Mb Upgrage kit for the ST(F/M)", ie what ur looking for, is listed at £99..... Bit steep....