You mentioned RP's Shadowrun line. Iron Wind Metals still produces that line, plus they're all readily available on the secondary market. In fact, all of them except Lone Wolf are currently available on Ebay.
The Female Street Samurai on Rapier, 20-541, $9 USD at IWM, usually goes for $7 USD-$10 on Ebay. She's more Asian than traditional biker-chick, but she's available and easily re-armed with a pistol, chainsaw, etc.
The Go-Gangers Gang Members on Bikes, 20-507, gives you a male and female biker, on foot and mounted, and goes for $14
USD at IWM, $10
USD-$15 on Ebay. Well, the girl isn't actually mounted, she's leaning on her bike. You could do an upper-body swap with the male rider, though.
The Go-Gangers Enforcers on Scorpion, 20-542, $11
USD and $10
USD-$15, are mounted on a cool tandem motorcycle. Again, a body swap for the male rider is called for.
There's also an Orc Biker on Scorpion (and on foot), 20-521, $12
USD and $10
USD-$15. Another body swap candidate.
Lone Wolf on Bike and on Foot, 20-581, $11
USD and $10
USD-$15. Another body swap.
Finally, there's the Ork and Dwarf on Heavy Trike, 20-543, $15
USD-$20. More body swaps.
That one line would give you a nice variety of bikes, and all in-scale with each other. There are also lots of Shadowrun females in the line to use for the body swaps, so they don't all look the same.
They're also not 28mm, more like large 25mm.