Hi Bill;
By the book, you want the front and rear counterweights to match the generation of the rotors ->
86-88 = Series4 (S4),
89-92= S5,
93-95= S6,  S5 and S6 are supposedly the same.

rear CW - a few quick searches will turn up more - going rate ~$50-60.  Dealer probably 2.5 times that.
http://www.rx7club.com/showthread.php?p=8911374

front CW:
The S4 and S5 front CW are different although they look very similar.
http://cgi.ebay.com/ebaymotors/Mazda-RX7-S5-Counterweight-Engine-Eccentric-Shaft-RX-7_W0QQcmdZViewItemQQcategoryZ33616QQihZ006QQitemZ160311630420QQrdZ1QQsspagenameZWDVW
http://cgi.ebay.com/ebaymotors/mazda-rx-7-rx7-13b-89-91-front-eshaft-counterweight_W0QQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247QQcategoryZ6763QQihZ010QQitemZ200248756262QQrdZ1QQsspagenameZWD1V
http://cgi.ebay.com/ebaymotors/New-13B-Front-Counterweight-RX7-89-91_W0QQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247QQcategoryZ33616QQihZ011QQitemZ320217523875QQrdZ1QQsspagenameZWD1V

Cheers
Cary

PS - more than a few engines have been assembled without being told this - and they ran just fine.  Although, best to try to match.
You should have nothing else to worry about your assembly.  Are you going to give it a good street port?