Last I checked, Vesalia still has some ACA 030/41 mhz cards left, with 64 megs of RAM.
I have the 56 mhz ACA card and it's outstanding, though it does work better with 3.1 ROM's in your amiga.
Otherwise it's ebay or amibay, or ask in the sales forum here. You can probably find something like an 030/25 with 8 megs of RAM for about a hundred bucks. A plain RAM expansion might be a tad cheaper, but the difference won't be that great and having a bit of an extra speed boost is nice in any event.